U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5127

Prepared in cooperation with the Federal Emergency Management Agency

Flood Study of the Suncook River in Epsom,

Pembroke, and Allenstown, New Hampshire, 2009
Cover. Photograph looking downstream at area of Suncook River avulsion, Epsom, New Hampshire. 

(Photograph taken on June 18, 2008)
Flood Study of the Suncook River in 

Epsom, Pembroke, and Allenstown, 

New Hampshire, 2009

By Robert H. Flynn

Prepared in cooperation with the Federal Emergency Management Agency

Scientific Investigations Report 2010–5127

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Flynn, R.H., 2010, Flood study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009: 

U.S. Geological Survey Scientific Investigations Report 2010–5127, 64 p. (Also available at http://pubs.usgs.gov/

sir/2010/5127.)
iii

Acknowledgments

The author would like to express appreciation to the following U.S. Geological Survey personnel 

who assisted with collection of data in this report—Richard Kiah, Scott Olson, Thor Smith, 

James Degnan, Chandlee Keirstead, Heather Manzi, and Glenn Berwick. 

The author would like to acknowledge that the cost of generating the 1- and 4-ft contour 

interval data used in the creation of the flood mapping for this study was funded through a 

New Hampshire Department of Safety-Bureau of Emergency Management grant (Emergency 

Management Performance Grant) with in-kind matching funding provided by the New Hampshire 

Department of Environmental Services through its dam-maintenance efforts in the Suncook 

River watershed.
THIS PAGE INTENTIONALLY LEFT BLANK
v

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................4

Description of the Study Area ............................................................................................................4

Flood Mapping and Flood-Plain Management Applications .........................................................6

Methods Used to Determine Flood Data....................................................................................................6

Field Data Collection.............................................................................................................................7

Indirect Determination of Peak Flow for the Suncook River Flood of April 2007 .......................7

Hydrologic Analyses.............................................................................................................................8

Hydraulic Analyses...............................................................................................................................8

Flood Elevations .............................................................................................................................................9

Summary........................................................................................................................................................10

References Cited..........................................................................................................................................10

Appendix 1. Suncook River 10-, 2-, 1- and 0.2-Percent Annual Exceedance Probability Flood 

Profiles (10-, 50-, 100-, and 500-Year Recurrence-Interval Floods, respectively)................25

Appendix 2. Suncook River April 2007 Flood and 50-, 20-, and 4-Percent Annual Exceedance 

Probability Flood Profiles (2-, 5-, and 25-Year Recurrence-Interval Floods, respectively) 39

Appendix 3. Elevation Reference Marks ..............................................................................................53

Appendix 4. Suncook River Flood Map of the Floodway, and the 100- and 500-Year Flood from 

the Confluence with the Merrimack River to the Upstream Corporate Limit of Epsom, 

New Hampshire..............................................................................................................................55

Figures

1. Map showing extent of the flood study of the Suncook River in Epsom, Pembroke, 

and Allenstown, NH......................................................................................................................2

2. Pre-avulsion aerial imagery with the new and abandoned channels of the May 2006 

Suncook River avulsion, Epsom, NH..........................................................................................3

3. Map showing location of the Suncook River streamgage at North Chichester, NH 

(01089500), and indirect discharge locations for the determination of the April 2007 

flood peak on the Suncook River ...............................................................................................5

Tables

1. Drainage areas and peak recurrence-interval flows for selected locations on the 

Suncook River in Epsom, Pembroke, and Allenstown, NH....................................................9

2. Cross sections and the 10-, 50-, 100-, and 500-year recurrence-interval flood data 

for Suncook River in Epsom, Pembroke, and Allenstown, NH............................................12

3. Cross sections and the 2-, 5-, 25-year recurrence-interval and April 2007 flood data 

for Suncook River in Epsom, Pembroke, and Allenstown, NH............................................15

4. Surveyed and HEC-RAS-modeled April 2007 elevations of flood high-water marks 

for the Suncook River, NH.........................................................................................................18

5. 1978 FIS and 2009 flood study 100-year flood elevations for selected locations in the 

Suncook River in Epsom, Pembroke, and Allenstown, NH..................................................19

6. Cross sections, 100-year recurrence-interval, and floodway data for Suncook River 

in Epsom, Pembroke, and Allenstown, NH.............................................................................20
vi

Conversion Factors, Datum, and Abbreviations

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2)

Volume

cubic yard (yd3) 0.7646 cubic meter (m3)

Flow rate

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F–32)/1.8

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

OTHER ABBREVIATIONS USED IN REPORT

ADCP Acoustic Doppler Current Profiler

FEMA Federal Emergency Management Agency

FIRM Flood Insurance Rate Map

FIS Flood Insurance Study

GPS Global Positioning System

HEC-RAS Hydrologic Engineering Center River Analysis System

NFIP National Flood Insurance Program

NHDOT New Hampshire Department of Transportation

NWIS National Water Information System

USACE U.S. Army Corps of Engineers

USGS U.S. Geological Survey
Abstract

On May 15, 2006, a breach in the riverbank caused an 

avulsion in the Suncook River in Epsom, NH. The breach 

in the riverbank and subsequent avulsion changed the established flood zones along the Suncook River; therefore, a new 

flood study was needed to reflect this change and aid in flood 

recovery and restoration. For this flood study, the hydrologic 

and hydraulic analyses for the Suncook River were conducted 

by the U.S. Geological Survey, in cooperation with the Federal 

Emergency Management Agency.

This report presents water-surface elevations and profiles 

determined using the U.S. Army Corps of Engineers onedimensional Hydrologic Engineering Center River Analysis 

System model, also known as HEC-RAS. Steady-state watersurface profiles were developed for the Suncook River from 

its confluence with the Merrimack River in the Village of 

Suncook (in Allenstown and Pembroke, NH) to the upstream 

corporate limit of the town of Epsom, NH (approximately 

15.9 river miles). Floods of magnitudes that are expected to 

be equaled or exceeded once on the average during any 

2-, 5-, 10-, 25-, 50-, 100-, or 500-year period (recurrence 

interval) were modeled using HEC-RAS. These flood events 

are referred to as the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year 

floods and have a 50-, 20-, 10-, 4-, 2-, 1-, and 0.2-percent 

chance, respectively, of being equaled or exceeded during 

any year. The 10-, 50-, 100-, and 500-year flood events are 

important for flood-plain management, determination of floodinsurance rates, and design of structures such as bridges and 

culverts. The analyses in this study reflect flooding potentials 

that are based on existing conditions in the communities of 

Epsom, Pembroke, and Allenstown at the time of completion 

of this study (2009). Changes in the 100-year recurrenceinterval flood elevation from the 1979 flood study were typically less than 2 feet with the exception of a location 900 feet 

upstream from the avulsion that, because of backwater from 

the dams in the abandoned channel, was 12 feet higher in the 

1979 flood study than in this study.

Introduction

The Suncook River drains a group of lakes to the south 

of Lake Winnipesaukee and flows for approximately 30 mi 

to the Merrimack River at a location between Concord and 

Manchester, NH (fig. 1). Floods typically occur along the 

Suncook and Little Suncook Rivers during the spring, fall, 

and winter. Some of the most severe flooding in early spring 

is a result of snowmelt and heavy rains in conjunction with 

river ice. Major flooding in the fall is a result of heavy rainfall 

associated with tropical storms. Large thunderstorms can 

result in rapid runoff and flooding in the downstream reaches 

of small streams.

Two floods in March 1936 resulted in the largest flood 

of record for the Suncook River. A combination of saturated 

ground, warm temperatures, snowmelt, and heavy rains from 

the second flood, following the filling of lakes and reservoirs 

and high river flows from the first flood, resulted in a peak 

flow of 12,900 ft3/s at the U.S. Geological Survey (USGS) 

streamgage Suncook River at Depot Road in North Chichester, 

NH (streamgage number 01089500), hereinafter referred to 

as the Suncook River streamgage. The March 1936 peak flow 

was determined to be between a 100- and 500-year event 

(Flynn, 2008; Olson, 2007) and remains the largest flood of 

record at the Suncook River streamgage. 

From May 13 to 16, 2006, central and southern New 

Hampshire were flooded as a result of rainfall in excess of 

8.8 in. (measured in Concord, NH) over a 4-day period. 

During the storm, flooding, along with streambed and 

streambank erosion and deposition, was substantial at 

several locations in the state. As a result of the flood damage, 

a presidential disaster declaration was made on May 25, 

2006, for the New Hampshire counties of Rockingham, 

Hillsborough, Strafford, Merrimack, Belknap, Carroll, 

and Grafton. In response to this declaration, the USGS, in 

cooperation with the Federal Emergency Management Agency 

(FEMA), measured or computed flood data at 65 streamgages 

within the disaster declaration area (Olson, 2007). On the basis 

of surveyed high-water marks, the peak flow at the Suncook 

River streamgage for the May 2006 event was determined to 

be 7,600 ft3/s (Olson, 2007), the third largest flood of record at 

this streamgage. This flow was determined from the Suncook 

Flood Study of the Suncook River in Epsom, Pembroke, and 

Allenstown, New Hampshire, 2009

By Robert H. Flynn
2 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

ROCKINGHAM

STRAFFORD

BELKNAP

GRAFTON

SULLIVAN

CHESHIRE

MERRIMACK

HILLSBOROUGH

EXPLANATION

CARROLL

Base from U.S. Geological Survey digital line graphs, 1:24,000, 1990-94 and

 National Hydrography Dataset, 1:24,000, 1999

71°

44°

43°

45°

Concord

Manchester

Maine

Massachusetts

Vermont

CANADA

72°

New

Hampshire

Danbury Hill

Andover

Franklin

Northfield

Canterbury Loudon

Pittsfield

Chichester

Epsom

Allenstown Hooksett Dunbarton

Bow

Hopkinton Henniker

Bradford

Newbury

Sutton

Warner

Webster

Salisbury

Wilmot

New

London

Boscawen

Pembroke

Concord

MASSACHUSETTS

VERMONT

0

0

25 MILES

25 KILOMETERS 

72 ̊ 71 ̊

43 ̊

43 ̊30”

Merrimack River

Soucook River

Suncook River

Merrimack River

Suncook River study area

NEW HAMPSHIRE

NEW HAMPSHIRE

MAINE

Winnipesaukee

Lake

Chichester

Village of

Suncook

Figure 1. Extent of the flood study of the Suncook River in Epsom, Pembroke, and Allenstown, NH.
Introduction 3

River streamgage rating curve (stage/flow relation) that was 

extended above 4,500 ft3/s on the basis of a slope-area and 

contracted-opening measurement at 15.27 ft for the flood peak 

of 12,900 ft3/s on March 19, 1936. 

On May 15, 2006, a breach in the riverbank south of U.S. 

Route 4 and east of State Route 28 created an avulsion in the 

Suncook River in Epsom, NH (fig. 2). An avulsion refers to 

the abandonment of a river channel and the formation of a 

new river channel. Prior to the breach, the Suncook River 

flowed southward around Bear Island in Epsom, NH, in two 

stream reaches that joined to the west of Round Pond. The 

breach during the May 2006 flood created a new channel 

upstream from Bear Island and two dams at the north end of 

Bear Island, with the new channel rejoining the old channel 

to the east of Bear Island. After May 16, there was no flow 

in the western and northeastern reach of the Suncook River 

around Bear Island. Instead, the Suncook River created a path 

through a gravel and sand pit resulting in a channel to the east 

of its former location (fig. 2). The new channel through the 

former gravel and sand pit created by the breach of May 15, 

2006, was 0.42 mi (2,220 ft) in length, whereas the abandoned 

primary east channel was 0.87 mi (4,595 ft) in length. The 

abandoned west channel was 1.14 mi (6,040 ft) in length; the 

total length of abandoned channel was 2.01 mi. The length 

of the new channel of the Suncook River from the location 

upstream from the former confluence with the abandoned 

channel (upstream from the avulsion), through the former 

sand and gravel pit, back into the east channel and to the 

downstream location of the confluence at the abandoned 

west channel (southern end of Bear Island) was 1.02 mi 

(5,407 ft). As a result of the reduction in total stream length, 

from 2.01 to 1.02 mi, the average gradient of the Suncook 

River increased approximately 40 percent between the 

upstream and downstream locations of the abandoned channels. Because of this increased gradient, the average velocity 

of the river will increase, which is expected to increase the 

ability of the river to erode the riverbed and banks.

The breach and subsequent avulsion also affected 

sediment induction into the Suncook River because of the 

new path of the river through the sand and gravel pit. It was 

estimated that 150,000 yd3 of sediment were introduced into 

the Suncook River as a result of the new channel (Wittkop, 

Base map from New Hampshire Department of Transportation,

2006, 2005 1-foot color aerial photos, southeastern New Hampshire

Round Pond

 Bear 

Island

71°22'30" 71°22' 71°21'30" 71°21'

43°13'30"

43°13'

43°12'30"

S uncook River

EXPLANATION

Abandoned, pre-avulsion channel

New, post-avulsion channel

28

4

Suncook Rive r

250 500 METERS

2,000 FEET

0

0 1,000

Figure 2. Pre-avulsion aerial imagery with the new and abandoned channels of the May 2006 

Suncook River avulsion, Epsom, NH.
4 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

2006). Sheets of silt and sand, up to 5 ft thick, were deposited 

downstream from the avulsion channel, and this deposition is 

expected to decrease channel depth and increase the frequency 

of overbank flooding. In the avulsion area and upstream, 

downcutting of the stream continues to occur because of the 

increased gradient of the Suncook River and the convergence 

of flow. 

From April 16 to 18, 2007, southeastern New Hampshire 

experienced severe flooding as a result of a storm that stalled 

off the New England coast and deposited, in some locations, 

more than 7 in. of rainfall on an existing snowpack. As a result 

of this flood event, a presidential disaster declaration was 

made for Grafton, Hillsborough, Merrimack, Rockingham, 

and Strafford Counties in New Hampshire. On May 10, 2007, 

Belknap County was added to the disaster declaration. In 

response to this declaration, the USGS, in cooperation with 

FEMA, measured or computed flood data at 57 streamgages 

and 4 ungaged locations within and adjacent to the disaster 

areas (Flynn, 2008). On the basis of hydraulic models 

calibrated with surveyed high-water marks, the peak flow at 

the USGS Suncook River streamgage for the April 2007 event 

was determined by indirect methods to be 10,600 ft3/s and 

to have a recurrence interval of between 100 and 500 years 

(Flynn, 2008). This flood peak was the second largest flood 

of record at this streamgage. The May 2006 flood peak was 

subsequently estimated to be 6,600 ft3/s on the basis of the 

original rating curve extended with the April 2007 peak. The 

2006 peak flow was determined to have a recurrence interval 

of between 25 and 50 years. The May 2006 and April 2007 

flood peaks were listed by the USGS as “extremes outside 

period of record.” 

Although the recurrence interval represents the longterm average period between floods of a specific magnitude, 

floods can recur at short intervals and within the same year. 

The probability of experiencing a flood of a given magnitude 

increases when periods greater than 1 year are considered. For 

example, the probability of a flood that equals or exceeds the 

100-year flood (1-percent chance of annual exceedance) in any 

50-year period is approximately 40 percent (4 in 10), and for 

any 90-year period, the probability increases to approximately 

60 percent (6 in 10). The analyses reported herein reflect 

flooding potentials that are based on conditions in the communities at the time this study was completed (2009). 

The breach in the riverbank and subsequent avulsion 

changed the established flood zones along the Suncook River 

as defined by the U.S. Department of Housing and Urban 

Development Federal Insurance Administration (1978a, 

1978b, 1978c); therefore, new flood maps were needed to 

reflect this change to the impact of the Suncook River on 

property and homes in the communities along the river. The 

current effective FEMA Flood Insurance Rate Maps (FIRMs) 

and Flood Insurance Studies (FISs) for Allenstown (U.S. 

Department of Housing and Urban Development Federal 

Insurance Administration, 1978a), Epsom (U.S. Department 

of Housing and Urban Development Federal Insurance 

Administration, 1978b) and Pembroke (U.S. Department 

of Housing and Urban Development Federal Insurance 

Administration, 1978c) contain detailed 100- and 500-year 

flood-hazard areas that are no longer valid because of the 

breach and updated hydrology and topographic data, and 

need to be updated. This flood study to update the 100- and 

500-year flood elevations was conducted by the USGS in 

cooperation with FEMA. Updated elevations and flood-plain 

maps for the Suncook River can be used by state and local 

officials to guide the redevelopment and replacement of 

damaged infrastructure and the allocation of state and federal 

recovery resources, and to ensure that effective mitigation 

measures are in place to avoid future flood losses.

Purpose and Scope

This report provides updated flood elevations for the 

Suncook River as a result of the May 2006 and April 2007 

floods and the May 2006 avulsion. This study area included 

the Suncook River from the confluence with the Merrimack 

River to the upstream corporate limit of the town of Epsom 

(15.9 mi) in Merrimack County, NH (fig. 1). The results of this 

study may be used to update the previous FISs and FIRMs for 

the towns of Allenstown, Epsom, and Pembroke, NH (U.S. 

Department of Housing and Urban Development Federal 

Insurance Administration, 1978a, 1978b, 1978c; respectively), 

and may be used to update existing flood-plain regulations 

as part of the National Flood Insurance Program (NFIP). The 

information also can be used by local and regional planners to 

further promote sound land use and flood-plain development.

Floods that are expected to be equaled or exceeded 

once on the average during any 10-, 50-, 100-, or 500-year 

period (recurrence interval) have been recognized as having 

special importance for flood-plain management and for floodinsurance rates and were modeled in this study. These events, 

commonly termed the 10-, 50-, 100-, and 500-year floods, 

have a 10-, 2-, 1-, and 0.2-percent chance, respectively, of 

being equaled or exceeded during any year. In addition, the 

2-, 5-, and 25-year (recurrence interval) flood events and the 

peak flood of April 16–18, 2007, were modeled in this study. 

The 2-, 5-, and 25-year floods have a 50-, 20-, and 4-percent 

chance, respectively, of being equaled or exceeded during 

any year.

Description of the Study Area

The Suncook River flows through the towns of Epsom, 

Pembroke, and Allenstown, NH, in southeastern Merrimack 

County, which is in south-central New Hampshire (fig. 1). 

State Route 28 is parallel to the Suncook River and connects 

the towns of Epsom, Pembroke, and Allenstown. The Suncook 

River originates to the south of Lake Winnipesaukee, is 

approximately 39 mi long, and flows in a southerly direction. 

Its confluence with the Merrimack River is in the Village 

of Suncook, NH, which is located within the towns of 

Allenstown and Pembroke, NH. The drainage area is 154 mi2
Introduction 5

at the USGS Suncook River streamgage in North Chichester, 

NH, and 255.7 mi2 at the confluence of the Suncook River 

with the Merrimack River in the Village of Suncook, NH. 

The town of Epsom comprises 34.5 mi2 of land area 

and 0.1 mi2 of inland water area. The population of Epsom 

was 4,567 in 2007 (New Hampshire Employment Security, 

Economic and Labor Market Information Bureau, 2009b) or 

approximately 132 people per square mile. The population of 

Epsom increased 66 percent between 1980 and 2007. 

The town of Pembroke includes the Villages of North 

Pembroke and Suncook and comprises 22.6 mi2 of land area 

and 0.2 mi2 of inland water area. The population of Pembroke 

was 7,353 in 2007 (New Hampshire Employment Security, 

Economic and Labor Market Information Bureau, 2009c) or 

approximately 322 people per square mile. The population of 

Pembroke increased 51 percent between 1980 and 2007.

The town of Allenstown includes the Village of Suncook 

and comprises 20.5 mi2 of land area and 0.1 mi2 of inland 

water area. The population of Allenstown was 4,943 in 2007 

(New Hampshire Employment Security, Economic and Labor 

Market Information Bureau, 2009a) or approximately 240 people per square mile. The population of Allenstown increased 

12 percent between 1980 and 2007.

The Little Suncook River, a tributary to the Suncook 

River, originates east of Epsom at Northwood Lake and flows 

westerly to its confluence with the Suncook River downstream 

from U.S. Route 4 (fig. 3). The drainage area at the mouth 

of the Little Suncook River is approximately 41 mi2 (U.S. 

Department of Housing and Urban Development Federal 

Insurance Administration, 1978b). Numerous other small 

streams flow into these two rivers. In general, the flood plains 

along the Suncook and Little Suncook Rivers are sparsely 

developed with the exception of areas near the intersection of 

the Suncook River and U.S. Route 4, the intersection of the 

Suncook River and U.S. Route 3, downstream through the 

Village of Suncook, and near Gossville and Epsom along the 

Little Suncook River.

The prevailing winds in Epsom, Pembroke, and 

Allenstown are from the northwest and result in cold, dry air 

in the winter and cool, dry air in the summer. Strong winds 

from the south occur in July and August, and winds from 

the east typically coincide with summer and winter storms 

(National Climate Data Center, 2008). Precipitation falls on 

an average of 1 day out of 3 during the year; the frequency 

is slightly higher for April and May and slightly lower for 

August to October. Easterly winds and especially northeasterly 

28

4

MERRIMACK

COUNTY

Merrimack River

Soucook River

Suncook River

71°30'

43°00’

EXPLANATION

Base from U.S. Geological Survey Digital Line Graphs,

1:24,000, 1990−94 and National Hydrography Dataset,

1:24,000, 1999

71°15'

43°15'

U.S. Geological Survey Suncook River streamgage

at North Chichester, NH (01089500)

County boundary

Indirect discharge-measurement sites

Northwood

Lake Littl e Su ncookRi v er

Bear Brook

0 5 10 MILES

0 5 10 KILOMETERS

Suncook River at

Websters Mill Road,

Chichester/Pittsfield

Suncook

River at North

Chichester

Suncook River at

Short Falls Road, Epsom

Figure 3. Location of the Suncook River streamgage at North Chichester, NH (01089500), and indirect 

discharge locations for the determination of the April 2007 flood peak on the Suncook River.
6 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

winds are associated with the more substantial rains and 

heavier snowfalls. Snow cover typically lasts from midDecember until the last week of March, although bare 

ground is not a rarity in the winter. Rain, sleet, or freezing 

rain may also occur during the winter (National Climate Data 

Center, 2008). 

The average daily maximum and minimum temperatures 

for July in Concord, NH, are 82.2 and 57.2°F, respectively. 

The average daily maximum and minimum temperatures for 

January in Concord are 31.1 and 10.7°F, respectively. Average 

annual precipitation in Concord for 1979–2008 (rain, snow, 

and sleet) is 37.9 in. Average annual snowfall in Concord for 

1979–2008 is 63.8 in. (National Climate Data Center, 2008). 

The topography of Epsom, Pembroke, and Allenstown is 

hilly and characterized by stratified and unstratified material 

transported by and deposited from retreating glacial ice. The 

flood plains adjacent to the rivers are comprised of alluvial 

silt overlying glacial outwash. The predominant soil group 

is composed primarily of well-drained sandy loam in glacial 

till. Kames, terraces, deltas, and outwash plains are glacial 

landforms common to this area. Elevations range from 192 ft 

(NGVD 29) at the confluence of the Suncook and Merrimack 

Rivers to 1,413 ft (NGVD 29) at the summit of Fort Mountain 

in Epsom. Wetland areas throughout the three towns serve as 

the headwaters for many smaller streams.

The Suncook River has no major flood-control structures. 

The Pittsfield Mill Dams in Pittsfield, NH, Buck Street Dam in 

Pembroke, NH, and Webster, Pembroke, and China Mill Dams 

in the Village of Suncook are all run-of-river dams and do not 

act as flood-control dams. The Webster, Pembroke, and China 

Mill Dams are used for hydroelectric power generation. These 

dams do little to attenuate major flood peaks.

Flood Mapping and Flood-Plain Management 

Applications

The 100- and 500-year flood-plain and floodway 

boundaries have been delineated using the flood elevations 

determined at 160 cross sections along the river. To provide 

a national standard without regional discrimination, the 

1-percent annual exceedance probability (100-year) flood 

has been adopted by FEMA as the base flood for purposes of 

flood-plain management. The 0.2-percent annual exceedance 

probability (500-year) flood is used to indicate additional areas 

of flood risk in the community.

The floodway is the channel of a stream plus any adjacent 

flood-plain areas that must be kept free of encroachment so 

that the 100-year recurrence-interval flood can be carried 

without substantial increases in flood heights. Minimum 

federal standards limit such increases to 1.0 ft provided that 

hazardous velocities are not produced. The floodway presented 

in this study was computed for certain stream segments on 

the basis of equal conveyance reduction from each side of the 

flood plain. Floodway widths were computed at cross sections. 

Encroachment on flood plains, such as structures and fill, 

reduces flood-carrying capacity, increases flood heights and 

velocities, and increases flood hazards in areas beyond the 

encroachment itself. One aspect of flood-plain management 

involves balancing the economic gain from flood-plain 

development against the resulting increase in flood hazard. 

For purposes of the NFIP, delineation of a floodway can assist 

local communities in this aspect of flood-plain management. 

The floodways in this study are presented as a minimum 

standard that can be adopted directly by the towns. The area 

between the floodway and 100-year flood-plain boundaries is 

termed the floodway fringe. The floodway fringe encompasses 

that part of the flood plain that could be completely obstructed 

without increasing the water-surface elevation of the 100-year 

flood by more than 1.0 ft at any point. 

The 1-ft contour-interval data (Eastern Topographics, 

2007) were created for the reach of the Suncook River from 

U.S. Route 4 to approximately 0.25 mi downstream from 

the Short Falls Road bridge. These 1-ft contour-interval data 

extended out to 660 ft on either side of this reach. The 4-ft 

contour-interval data (Eastern Topographics, 2008) were created for the reach of the Suncook River from the Short Falls 

Road bridge to the confluence with the Merrimack River. 

These 4-ft contour-interval data extended out to 1,000 ft on 

either side of the Suncook River. In areas beyond the extent 

of the 1- and 4-ft contour-interval data, 10- and 20-ft contour 

interval USGS Digital Orthophoto Quadrangles produced 

at a scale of 1:24,000 referenced to the horizontal datum of 

NAD 83 and the vertical datum of NGVD 29 (U.S. Geological 

Survey, 1967a, 1967b, 1968, 1981) were used to delineate the 

extent of flooding on the 100- and 500-year flood map for the 

Suncook River. 

The base map for the delineation of the floodway and the 

100- and 500-year floods (appendix 4) was created using black 

and white aerial photographic imagery collected by Eastern 

Topographics (2007) in May 2007 (post-avulsion) along the 

Suncook River and 1-ft-resolution color aerial photographic 

imagery of southeastern New Hampshire (New Hampshire 

Department of Transportation, 2006) collected for the New 

Hampshire Department of Transportation (NHDOT) in May 

2005 (pre-avulsion) for areas outside the extent of the Eastern 

Topographics (2007) imagery. 

Methods Used to Determine Flood Data

Hydrologic and hydraulic analyses were used to 

determine the flood data required for this flood study of the 

Suncook River. The U.S. Army Corps of Engineers (USACE) 

step-backwater flow model, Hydrologic Engineering Center 

River Analysis System model (HEC-RAS) (U.S. Army 

Corps of Engineers, 2008a), was used for the determination 

of water-surface elevations at surveyed cross sections and, 

subsequently, flood profiles for the selected recurrence-interval 

floods along the study reach. The water-surface elevations 

determined with HEC-RAS were used to create the flood map 
Methods Used to Determine Flood Data 7

delineating the extent of the 100- and 500-year floods. Input 

for the HEC-RAS model required the collection of hydraulic 

and hydrologic field data to characterize channel geometry and 

flow. Hydraulic data included river cross-section surveys at 

160 locations and structural surveys at 7 bridges and 

5 inline (dam) structures. Hydrologic data included analysis 

of streamflow data from the Suncook River streamgage that 

incorporated peak-flow data determined by indirect methods in 

2006 and 2007 (Olson, 2007; Flynn, 2008; respectively). The 

Suncook River HEC-RAS model was calibrated to the flood 

of April 2007 using high-water-mark data collected after that 

flood along with the flood peak flow determined by indirect 

methods at the Suncook River streamgage.

All FISs and FIRMs are referenced to a specific 

vertical datum. The vertical datum provides a starting point 

against which flood, ground, and structure elevations can 

be referenced and compared. All structure, ground, and 

flood elevations given in this flood-study report and map are 

referenced to the NGVD 29, unless otherwise noted. The 

coordinate system used for the production of the digital flood 

map is New Hampshire State Plane referenced to the NAD 83 

and NGVD 29. A description of the reference marks used in 

this study is presented in appendix 3. 

Field Data Collection

After the April 16–18, 2007, flood, high-water-mark 

information, indicating peak flood stage, was collected 

upstream from, downstream from, and at the former USGS 

Suncook River streamgage (Flynn, 2008). The high-water 

marks consisted of debris lines, wash lines, seed lines, or 

mud/silt lines left behind by the peak water elevation during 

the flood. The peak-flood-stage data at flagged high-water 

marks and field-surveyed cross sections were referenced to 

NGVD 29 through closed level-loop surveying techniques 

from either a known reference datum or from a reference 

datum established using a Leica System 1200 Global 

Positioning System (GPS) with 3-mm (0.01-ft) horizontal 

accuracy and 10-mm (0.03-ft) vertical accuracy in static mode 

(Leica Geosystems, 2008). 

Cross sections for the 15.9-mi study reach of the Suncook 

River were from field surveys conducted in the summer 

of 2008. Valley and below-water cross-section data for the 

Suncook River were obtained from field measurements. 

Additional valley cross-section data were obtained using 

1-ft (Eastern Topographics, 2007) and 4-ft (Eastern 

Topographics, 2008) contour-interval data created from aerial 

photogrammetry collected in May 2007 and April 2008. All 

bridges, dams, and culverts were field-surveyed to obtain 

elevation data and structural geometry. Bridge, dam, and 

culvert cross-section data were collected at close intervals 

upstream and downstream from bridges and dams in order to 

compute the potential backwater effects of these structures. 

Between the Short Falls Road bridge and the Buck Street 

Dams, cross-section and profile data were collected using a 

GPS in conjunction with an Acoustic Doppler Current Profiler 

(ADCP) (Mueller and Wagner, 2009). 

Indirect Determination of Peak Flow for the 

Suncook River Flood of April 2007

The Suncook River streamgage was active from 1919 to 

1920, 1922 to 1927, and 1929 to 1977. Because the Suncook 

River streamgage was inactive from 1978 to October 2007, 

it was not active during the two peak-flow events in May 

2006 and April 2007 but was re-activated in November 2007. 

After the May 2006 and April 2007 floods, high-water-mark 

information was collected by the USGS at the Suncook River 

streamgage (Olson, 2007; Flynn, 2008). These data were used 

to estimate the flood peak flow for these events on the basis 

of the rating curve established for the streamgage for the 

period of record prior to 1977. Additional high-water-mark 

information was collected upstream and downstream from the 

Suncook River streamgage after the April 2007 flood 

(Flynn, 2008).

Initially, a peak flow of 15,000 ft3/s was extrapolated for 

the April 16–18, 2007, flood at the Suncook River streamgage 

on the basis of the high-water-mark data and the rating curve. 

Subsequently, determinations of indirect flow were made at 

locations upstream and downstream from the discontinued 

streamgage, and a flow of 10,600 ft3/s was determined to be 

the peak flow for the April 16–18, 2007, flood at the Suncook 

River streamgage (Flynn, 2008). This peak flow was estimated 

at this streamgage on the basis of the flow/drainage-area 

relations between two indirectly determined (Matthai, 1967) 

April 2007 flood-peak flows. The April 2007 flood-peak 

indirect flow determinations were made at the Websters Mill 

Road bridge in Chichester/Pittsfield and at the Short Falls 

Road bridge in Epsom (fig. 3), which have drainage areas 

of 137 and 210 mi2, respectively. The high-water marks 

upstream and downstream from each bridge, the bridgeopening geometry, and channel and flood-plain cross sections 

upstream and downstream from the bridge were surveyed, and 

the values were input to the HEC-RAS model. Flows were 

selected iteratively as input to the HEC-RAS model (U.S. 

Army Corps of Engineers, 2005) so that the resulting watersurface elevation output from the program matched the highwater marks surveyed upstream and downstream from each of 

the bridges. The upper end of the rating curve (stage/discharge 

relation) at the Suncook River streamgage has likely shifted 

since it was determined from the March 19, 1936, flood data 

when a flow of 12,900 ft3/s at a gage height of 15.27 ft was 

ascertained by slope-area and contracted-opening methods. 

The shift in the upper end of the stage/discharge rating curve 

is likely because of a greater amount of vegetation on the right 

overbank in 2007 than in 1936 (as seen in USGS photographs 

on file at the New Hampshire-Vermont Water Science Center) 

and the construction of a weir control along with the removal 

of a channel island downstream from the Suncook River 

streamgage in the summer of 1937.
8 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Hydrologic Analyses

Hydrologic analyses were used to establish the peak flowfrequency relations for the Suncook River. For this flood study, 

peak-flow frequencies for the Suncook River were estimated 

by Flynn (2008) using a log-Pearson Type III analysis 

described in Bulletin 17B “Guidelines for Determining Flood 

Flow Frequency” (U.S. Interagency Advisory Committee 

on Water Data, 1982). Bulletin 17B provides procedures for 

weighting station skews, for determining historical peaks, and 

for detecting and treating outliers and trends. Data from the 

USGS Suncook River streamgage were used for computing 

the frequency curve. The USGS computer program PeakFQ 

(U.S. Geological Survey, 2008a) was used to analyze peakflow frequency. The peak-flow data used as input to the 

PeakFQ program were retrieved from the National Water 

Information System (NWIS) (U.S. Geological Survey, 2008b). 

To determine the peak flow-frequency curves for selected 

Suncook River locations, the relation between drainage areas 

at the streamgage and the ungaged locations (equation 1) was 

determined and applied to the log-Pearson Type III peak flowfrequency curve determined at the Suncook River streamgage. 

Q/Qg = (A/Ag )n , (1)

where,

Q is the flow at the ungaged site, 

Qg is the flow at the USGS streamgage, 

A is the drainage area at the ungaged site, 

Ag is the drainage area at the USGS streamgage, 

and

n is the drainage-area adjustment value of 0.94.

The exponent n in equation 1 was determined through a 

drainage-area relation between two indirectly determined April 

2007 peak flows at locations upstream and downstream from 

the Suncook River streamgage (Flynn, 2008).

Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding were 

carried out to provide estimates of the elevations of floods 

at the selected recurrence intervals. The hydraulic analyses 

for this study were based on an assumption of unobstructed 

flow. As a result, the flood elevations shown in the profiles are 

considered valid if hydraulic structures remain unobstructed, 

operate properly, and do not fail. Starting water-surface 

elevations for the Suncook River HEC-RAS (U.S. Army 

Corps of Engineers, 2008a) flood model were based on 

normal-depth analysis, which is the depth at which flow is 

steady and hydraulic characteristics are uniform. The model 

computational procedure is based on the solution of the onedimensional energy equation (U.S. Army Corps of Engineers, 

2008b). Energy losses are evaluated by friction (Manning’s 

equation) and contraction and expansion. The momentum 

equation is used in situations where the water surface is 

rapidly varied. These situations include mixed flow-regime 

calculations (i.e., hydraulic jumps), hydraulics of bridges, and 

evaluating profiles at river confluences (stream junctions). 

In those areas where the analysis indicated supercritical flow 

conditions, critical depth was assumed for the flood elevation 

because of the inherent instability of supercritical flow. The 

Suncook River HEC-RAS flood model was calibrated using 

22 high-water marks flagged and surveyed along the 

Suncook River (table 4) following the flood of April 2007 

(Flynn, 2008).

Manning’s “n” roughness factors (Barnes, 1967; 

Arcement and Schneider, 1989), used in the hydraulic 

computations, were assigned on the basis of engineering 

judgment and field observations of the stream and flood-plain 

areas. The channel and overbank Manning’s “n” values for 

the Suncook River study reach ranged from 0.02 to 0.09 in the 

channel and from 0.02 to 0.12 on the channel overbanks. 

The Suncook River flood-study base-map elevations for 

the 100-year recurrence-interval flood elevations (also known 

as the base flood elevation or BFE) were compared to the 1978 

FIS 100-year recurrence-interval floods elevations at selected 

locations (table 5). It is important to note that the Suncook 

River channel elevations have changed in many locations 

downstream from the avulsion since the 1978 FISs due to 

several factors, including newer techniques and methods in the 

HEC-RAS step-backwater model. Other factors affecting the 

channel elevations and the delineation of flood boundaries for 

the selected recurrence-interval floods in this flood study are:

• The peak discharges for the selected recurrenceinterval floods have been updated since the 1978 FIS 

with additional flow data available at the Suncook 

River streamgage, 

• 1- and 4-ft contour-interval data were used for flood 

delineation in this study; the 1978 FIS used the bestavailable mapping of 20-ft contour-interval data, 

• More detailed field-survey information was collected 

at the structures and in the channel and a greater number of cross sections were surveyed in the field in 2008 

than for the 1978 FISs.
Flood Elevations 9

Flood Elevations

Suncook River flood-study drainage areas and peak flows 

adjusted for selected locations on the Suncook River that were 

used as input for the HEC-RAS model are given in table 1. 

Suncook River streamgage peak recurrence-interval flows 

were determined in a previous study (Flynn, 2008).

The resulting flood elevations from the HEC-RAS flood 

analyses for the Suncook River are shown in table 2 and 

appendix 1 for the 10-, 50-, 100-, and 500-year recurrenceinterval floods. Flood elevations are shown in table 3 and 

appendix 2 for the 2-, 5-, and 25-year recurrence-interval 

floods and for the flood of April 2007. Selected cross-section 

locations used in the hydraulic analyses are shown in the flood 

profiles in appendixes 1 and 2. Upstream and downstream 

geographical limits of the study area are indicated on the 

flood profiles.

The HEC-RAS model calibration of the April 2007 flood 

to the 22 surveyed high-water marks (table 4) yielded an average elevation difference of 0.30 ft (approximately 3.6 in.) and 

a standard deviation of 0.81. A comparison of the HEC-RAS 

model calibration of the April 2007 flood to the 13 surveyed 

high-water marks rated as “good” in table 4 yielded an average 

elevation difference of 0.14 ft (approximately 1.7 in.) and a 

standard deviation of 0.55. Discrepancies between model and 

field results may be due to model assumptions or in the case 

of field-surveyed high-water marks, a field elevation higher 

than that modeled may be due to debris in the river, which can 

cause a higher localized water surface. A field elevation lower 

than that modeled may be due to an elevation from a mark 

type (seed, wash, debris, or silt lines) carried to a lower elevation as the flood waters receded.

For the selected locations shown in table 5, the average 100-year recurrence-interval flood elevation was 0.28 ft 

(approximately 3.3 in.) lower in this 2009 flood study than in 

the 1978 FISs. Locations for comparison were selected where 

water-surface-elevation data were determined in both the 1978 

FISs and the current study. Selected table 5 BFEs for the 2009 

flood study are also shown in table 6.

The results of the water-surface computations are 

tabulated for selected cross sections for the Suncook River 

(table 6) with all increases in the 100-year recurrenceinterval flood elevation limited to a maximum of 1.0 ft. For 

areas between cross sections, the floodway boundaries were 

interpolated. The computed floodway is shown on the flood 

map in appendix 4. A listing of stream velocities at selected 

cross sections is provided in table 6. 

Table 1. Drainage areas and peak recurrence-interval flows for selected locations on the Suncook River in Epsom, Pembroke, and 

Allenstown, NH.

[mi2, square miles; ft3/s, cubic feet per second]

Location

Drainage 

area

(mi2)

Peak flow

(ft3/s)

2-year 5-year 10-year 25-year 50-year 100-year 500-year April 

2007

At Suncook River streamgage 154.4 2,230 3,550 4,680 6,430 7,990 9,820 15,300 10,600

At U.S. Route 4 in Epsom 161.2 2,320 3,700 4,870 6,690 8,330 10,200 15,900 11,000

Below Little Suncook River 200.9 2,850 4,550 5,990 8,230 10,240 12,600 19,600 13,600

At Short Falls Road in Epsom 210.1 2,980 4,740 6,250 8,590 10,700 13,100 20,400 14,200

Above Bear Brook 219.3 3,100 4,940 6,510 8,940 11,100 13,700 21,300 14,700

Below Bear Brook 236.4 3,330 5,310 6,990 9,590 11,900 14,700 22,800 15,800

At U.S. Route 28 bridge 240.1 3,370 5,380 7,090 9,730 12,100 14,900 23,200 16,100

At U.S. Route 3 in Suncook 255.2 3,570 5,700 7,510 10,300 12,800 15,700 24,500 17,000

At mouth of Suncook River 255.7 3,580 5,710 7,520 10,300 12,800 15,800 24,600 17,000
10 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Summary

Two rainfall events in May 2006 and April 2007 resulted 

in severe flooding of the Suncook River. More than 8.8 inches 

of precipitation from May 13 to 16, 2006, resulted in a flood 

that lead to an avulsion of a streambank and a new reach 

location for part of the Suncook River. More than 7 inches of 

precipitation from April 16 to 18, 2007, resulted in another 

flood. Both floods transported large quantities of gravel, sand, 

silt, and clay downstream. This 2009 flood study by the U.S. 

Geological Survey, in cooperation with the Federal Emergency 

Management Agency, was undertaken to help guide state and 

local officials in redevelopment and replacement of damaged 

infrastructure, allocation of state and federal recovery 

resources, and in ensuring that effective mitigation measures 

are in place to avoid future flood losses.

Water-surface elevations and profiles are presented in 

this report that were determined using the U.S. Army Corps 

of Engineers one-dimensional Hydrologic Engineering 

Center River Analysis System (HEC-RAS) model for the 

Suncook River from its confluence with the Merrimack River 

in the Village of Suncook, NH, to the upstream corporate 

limit of the town of Epson, NH (approximately 15.9 miles). 

Floods of magnitudes that are expected to be equaled or 

exceeded once on the average during any 10-, 50-, 100-, or 

500-year period (recurrence interval) were modeled using 

HEC-RAS because these floods are considered important for 

flood-plain management, determination of flood-insurance 

rates, and design of structures, such as bridges and culverts. 

In addition, floods of magnitudes that are expected to be 

equaled or exceeded once on the average during any 2-, 5-, 

and 25-year period (recurrence interval) and the April 2007 

flood were modeled using HEC-RAS. The analyses in this 

study reflect the flooding potentials based on conditions in the 

communities of Epsom, Pembroke, and Allenstown at the time 

of completion of this study (2009). Changes in the 100-year 

recurrence interval flood elevation from the 1979 flood study 

were typically less than 2 feet, with the exception of a location 

900 feet upstream from the avulsion which, due to backwater 

from the dams in the abandoned channel, was 12 feet higher in 

1979 than in the present study.

References Cited

Arcement, G.J., and Schneider, V.R., 1989, Guide for selecting 

Manning’s roughness coefficients for natural channels and 

flood plains: U.S. Geological Survey Water-Supply Paper 

2339, 69 p.

Barnes, H.H., Jr., 1967, Roughness characteristics of natural 

channels: U.S. Geological Survey Water-Supply Paper 

1849, 213 p.

Eastern Topographics, 2007, Photogrammetric ground control 

report for the Town of Epsom, NH, Suncook River area: 

Epsom, NH R10B/10343{3}-1, August 30, 2007, 

NGVD 29, 1-ft contour interval, scale 1:12,000.

Eastern Topographics, 2008, Photogrammetric ground control 

report for the Towns of Epsom and Allenstown, NH, 

Suncook River area: Epsom and Allenstown, NH 

R10A/10573, August 30, 2007, NGVD 29, 4-ft contour 

interval, scale 1:36,000.

Flynn, R.H., 2008, Flood of April 2007 in New Hampshire: 

U.S. Geological Survey Scientific Investigations Report 

2008–5120, 48 p. (Available at http://pubs.usgs.gov/

sir/2008/5120/.)

Leica Geosystems, 2008, Leica GPS1200, GPS1200 

Surveying Systems: accessed November 9, 2008, at 

http://www.leica-geosystems.com/en/Products-LeicaSystem-1200_4580.htm.

Matthai, H.F., 1967, Measurement of peak discharge at width 

contractions by indirect methods: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 3, 

chap. A4, 44 p.

Mueller, D.S., and Wagner, C.R., 2009, Measuring discharge 

with acoustic doppler current profilers from a moving boat: 

U.S. Geological Survey Techniques and Methods 3–A22, 

72 p. (Available at http://pubs.water.usgs.gov/tm3a22.)

National Climate Data Center, 2008, Local climatological 

data annual summary with comparative data Concord, 

New Hampshire (KCON): accessed April 22, 2009, at 

http://www1.ncdc.noaa.gov/pub/orders/FF3E95A7-6803-

492A-7054-C33BBF06B197.PDF.

New Hampshire Department of Transportation, 2006, 

2005 1-ft Color Aerial Imagery–Southeast NH, Imagery 

acquired at an elevation of approximately 9,800 ft. AMSL 

on May 11, 2005.

New Hampshire Employment Security, Economic and Labor 

Market Information Bureau, 2009a, Community profiles 

(Allenstown, NH): accessed April 22, 2009, at http://www.

nh.gov/nhes/elmi/htmlprofiles/pdfs/allenstown.pdf.

New Hampshire Employment Security, Economic and Labor 

Market Information Bureau, 2009b, Community profiles 

(Epsom, NH): accessed April 22, 2009, at http://www.

nh.gov/nhes/elmi/htmlprofiles/pdfs/epsom.pdf.

New Hampshire Employment Security, Economic and Labor 

Market Information Bureau, 2009c, Community profiles 

(Pembroke, NH): accessed April 22, 2009, at http://www.

nh.gov/nhes/elmi/htmlprofiles/pdfs/pembroke.pdf.

Olson, S.A., 2007, Flood of May 2006 in New Hampshire: 

U.S. Geological Survey Open-File Report 2007–1122, 32 p. 

(Available at http://pubs.water.usgs.gov/ofr2007-1122.)
References Cited 11

U.S. Army Corps of Engineers, 2005, HEC-RAS, 

version 3.1.3: Hydrologic Engineering Center, accessed 

September 2, 2007, at http://www.hec.usace.army.mil.

U.S. Army Corps of Engineers, 2008a, Hydrologic 

Engineering Center-River Analysis System (HEC-RAS): 

Davis, Calif., v. 4.0.0, March 2008.

U.S. Army Corps of Engineers, 2008b, Hydrologic Engineering Center-River Analysis System (HEC-RAS), HEC-RAS 

features: accessed March 29, 2009, at http://www.hec.

usace.army.mil/software/hec-ras/hecras-features.html.

U.S. Department of Housing and Urban Development Federal 

Insurance Administration, 1978a, Flood-Insurance Study, 

Town of Allenstown, Merrimack County, New Hampshire: 

Washington, D.C., 16 p.

U.S. Department of Housing and Urban Development Federal 

Insurance Administration, 1978b, Flood-Insurance Study, 

Town of Epsom, Merrimack County, New Hampshire: 

Washington, D.C., 19 p.

U.S. Department of Housing and Urban Development Federal 

Insurance Administration, 1978c, Flood-Insurance Study, 

Town of Pembroke, Merrimack County, New Hampshire: 

Washington, D.C., 17 p.

U.S. Geological Survey, 1967a, Photoinspected 1974, 

Gossville, New Hampshire, 7.5-minute series quadrangle 

map: U.S. Geological Survey Topographic Map, 10-ft 

contour interval, scale 1:24,000.

U.S. Geological Survey, 1967b, Photoinspected 1985, 

Suncook, New Hampshire, 7.5-minute series quadrangle 

map: U.S. Geological Survey Topographic Map, 10-ft 

contour interval, scale 1:24,000.

U.S. Geological Survey, 1968, Photorevised 1985, Manchester 

North, New Hampshire, 7.5-minute series quadrangle map: 

U.S. Geological Survey Topographic Map, 10-ft contour 

interval, scale 1:24,000.

U.S. Geological Survey, 1981, Pittsfield, New Hampshire, 

7.5-minute series quadrangle map: U.S. Geological Survey 

Topographic Map, 20-ft contour interval, scale 1:24,000.

U.S. Geological Survey, 2008a, Water resources application 

software, PeakFQ: accessed October 12, 2008, at 

http://water.usgs.gov/software/peakfq.html.

U.S. Geological Survey, 2008b, Peak streamflow for New 

Hampshire: National Water Information System, accessed 

October 12, 2008, at http://nwis.waterdata.usgs.gov/nh/

nwis/peak.

U.S. Interagency Advisory Committee on Water Data, 1982, 

Guidelines for determining flood flow frequency, Bulletin 

17-B of the Hydrology Subcommittee: Reston, Va., 

U.S. Geological Survey, Office of Water Data Coordination, 

183 p.

Wittkop, C., 2006, NH Geological Survey letter to the New 

Hampshire Department of Environmental Services entitled: 

Suncook River Initial Assessment, 2 p.
12 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Table 2. Cross sections and the 10-, 50-, 100-, and 500-year recurrence-interval flood data for Suncook River in Epsom, Pembroke, 

and Allenstown, NH.—Continued

[Location of cross sections A–DW are in appendix 4, where scale permits, and in appendix 1; ft, feet; NGVD 29, National Geodetic Vertical Datum of 1929]

Suncook River 

cross section

Cumulative 

distance upstream1

(ft)

Minimum channel 

elevation 

(ft above NGVD 29)

Annual exceedance probability flood elevation

(ft above NGVD 29)

10 percent 

(10-year)

2 percent 

(50-year)

1 percent 

(100-year)

0.2 percent 

(500-year)

A 0 181.57 2195.1 2197.0 2198.3 2205.0

B 477 183.54 2195.1 2197.0 2198.3 2205.0

C 804 184.00 2195.1 2197.0 2198.3 2205.0

D 1,681 196.62 201.59 202.98 203.68 205.49

E 2,043 198.43 206.50 209.69 211.77 220.33

F 2,193 204.12 209.56 211.36 212.46 220.28

G 2,507 214.79 233.46 236.30 237.27 239.38

H 3,079 219.73 233.26 235.84 236.61 237.91

I 3,401 242.10 256.21 261.07 263.51 273.85

J 3,467 242.20 256.47 261.10 263.70 274.04

K 3,880 250.58 260.26 263.19 265.16 274.38

L 3,934 251.54 260.59 263.58 265.50 274.59

M 4,433 258.46 266.27 268.42 269.44 273.05

N 4,507 259.63 267.43 269.59 270.61 273.09

O 4,640 263.45 278.41 280.21 281.07 282.93

P 4,886 265.02 278.51 280.46 281.41 283.61

Q 5,126 265.07 278.79 280.89 281.90 284.28

R 5,283 263.79 278.84 280.98 282.01 284.45

S 5,575 269.45 278.88 281.05 282.10 284.60

T 5,860 268.32 278.60 280.55 281.49 283.68

U 6,045 270.65 278.51 280.28 281.11 282.85

V 6,172 268.84 279.96 282.62 286.44 290.08

W 6,935 274.18 280.82 282.99 286.71 290.41

X 6,945 274.18 281.75 283.70 286.80 291.69

Y 7,981 270.26 284.85 287.76 289.45 292.70

Z 9,331 267.96 285.67 289.00 290.76 294.26

AA 10,614 270.63 285.95 289.30 291.05 294.58

AB 11,542 270.97 286.30 289.80 291.42 294.94

AC 12,282 269.33 286.49 290.02 291.64 295.21

AD 15,482 266.90 287.08 290.74 292.39 295.99

AE 17,581 267.70 287.58 291.01 292.58 296.06

AF 19,123 268.58 287.93 291.41 292.99 296.50

AG 22,592 271.35 288.56 291.96 293.56 297.21

AH 23,529 270.36 288.68 291.89 293.59 297.23

AI 24,763 272.96 289.32 292.28 293.64 297.26

AJ 25,387 273.31 289.51 292.32 293.67 297.27

AK 26,696 273.13 289.89 292.47 293.63 297.08

AL 27,674 275.27 290.25 293.11 294.31 297.60

AM 28,253 277.21 290.50 293.44 294.71 298.11

AN 28,443 279.58 290.76 293.83 295.13 298.54

AO 28,766 281.21 291.31 294.34 295.65 299.01

AP 28,851 279.83 291.59 294.55 295.82 299.12

AQ 29,063 283.13 296.36 298.76 300.71 303.25

AR 29,088 283.13 296.37 298.78 300.73 303.28
Table 2 13

Table 2. Cross sections and the 10-, 50-, 100-, and 500-year recurrence-interval flood data for Suncook River in Epsom, Pembroke, 

and Allenstown, NH.—Continued

[Location of cross sections A–DW are in appendix 4, where scale permits, and in appendix 1; ft, feet; NGVD 29, National Geodetic Vertical Datum of 1929]

Suncook River 

cross section

Cumulative 

distance upstream1

(ft)

Minimum channel 

elevation 

(ft above NGVD 29)

Annual exceedance probability flood elevation

(ft above NGVD 29)

10 percent 

(10-year)

2 percent 

(50-year)

1 percent 

(100-year)

0.2 percent 

(500-year)

AS 29,216 280.40 296.37 298.73 300.63 303.05

AT 29,346.5 279.21 296.42 298.90 300.95 303.80

AU 29,599.5 279.10 296.40 298.81 300.78 303.46

AV 30,714.5 279.32 296.51 298.99 300.93 303.57

AW 31,746.5 276.97 296.70 299.37 301.39 304.42

AX 33,141.5 276.47 296.89 299.58 301.55 304.58

AY 34,226.5 279.64 297.11 299.97 301.93 305.11

AZ 34,926.5 277.09 297.13 299.92 301.85 304.99

BA 35,562.5 278.13 297.46 300.41 302.33 305.61

BB 36,077.5 278.99 297.55 300.55 302.47 305.77

BC 36,612.5 275.28 297.77 300.83 302.73 306.06

BD 37,932.5 280.62 298.14 301.36 303.28 306.67

BE 38,425.5 279.70 298.33 301.53 303.41 306.80

BF 39,120.5 277.56 298.62 301.89 303.76 307.23

BG 39,860.5 277.90 298.72 301.99 303.85 307.38

BH 40,505.5 284.98 298.94 302.31 304.18 307.60

BI 41,065.5 285.20 299.20 302.52 304.36 307.80

BJ 41,898.5 278.65 299.39 302.67 304.48 307.90

BK 43,018.5 284.04 299.73 303.00 304.78 308.22

BL 43,740.5 279.70 300.06 303.39 305.17 308.68

BM 44,820.5 285.93 300.34 303.72 305.48 308.99

BN 45,160.5 286.48 300.56 304.07 305.87 309.39

BO 45,723.5 278.21 300.87 304.42 306.16 309.61

BP 46,355.5 286.15 300.97 304.51 306.24 309.76

BQ 46,730.5 280.10 301.26 304.93 306.70 310.26

BR 47,374.5 286.45 301.46 305.21 307.00 310.57

BS 48,052.5 283.65 301.59 305.27 307.03 310.54

BT 48,852.5 282.55 301.90 305.61 307.37 310.94

BU 50,157.5 285.85 302.47 306.22 307.96 311.58

BV 50,623.5 286.80 302.69 306.53 308.31 312.02

BW 50,828.5 271.36 302.76 306.60 308.38 312.09

BX 51,080.5 287.32 302.71 306.52 308.28 311.95

BY 51,304.5 290.47 303.22 307.30 309.19 313.69

BZ 51,974.5 289.81 303.84 308.01 309.95 314.56

CA 52,462.5 289.73 304.13 308.29 310.22 314.88

CB 53,599.5 289.59 304.27 308.39 310.32 314.97

CC 54,414.5 293.58 304.41 308.46 310.38 315.02

CD 55,868.5 292.55 304.51 308.50 310.41 315.04

CE 56,344.5 294.28 304.53 308.51 310.42 315.04

CF 56,879.5 294.58 304.57 308.52 310.43 315.05

CG 57,635.5 295.63 304.59 308.52 310.43 315.05

CH 58,326.5 296.89 304.66 308.54 310.44 315.06

CI 58,366.5 296.89 304.68 308.55 310.45 315.07
14 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Table 2. Cross sections and the 10-, 50-, 100-, and 500-year recurrence-interval flood data for Suncook River in Epsom, Pembroke, 

and Allenstown, NH.—Continued

[Location of cross sections A–DW are in appendix 4, where scale permits, and in appendix 1; ft, feet; NGVD 29, National Geodetic Vertical Datum of 1929]

Suncook River 

cross section

Cumulative 

distance upstream1

(ft)

Minimum channel 

elevation 

(ft above NGVD 29)

Annual exceedance probability flood elevation

(ft above NGVD 29)

10 percent 

(10-year)

2 percent 

(50-year)

1 percent 

(100-year)

0.2 percent 

(500-year)

CJ 59,098.5 296.17 304.80 308.58 310.47 315.08

CK 59,428.5 296.20 304.87 308.59 310.47 315.08

CL 59,963.5 296.75 305.38 308.72 310.55 315.12

CM 60,519.5 299.31 305.88 308.79 310.59 315.13

CN 61,159.5 298.38 306.02 308.68 310.58 315.10

CO 61,867.5 299.16 310.09 311.27 311.82 315.41

CP 62,546.5 299.17 311.20 312.77 313.45 316.16

CQ 63,034.5 299.49 311.98 313.89 314.70 317.03

CR 63,474.5 300.55 312.92 315.11 315.98 317.87

CS 63,794.5 301.23 313.53 315.93 316.81 318.59

CT 64,131.5 302.17 313.59 315.83 316.58 317.99

CU 64,399.5 303.38 313.93 316.30 317.13 318.72

CV 64,631.5 303.48 314.40 317.01 318.02 320.30

CW 64,892.5 304.21 314.55 317.22 318.27 320.80

CX 65,149.5 303.58 314.85 317.45 318.52 321.07

CY 65,882.5 306.19 316.01 318.57 319.56 322.02

CZ 65,892.5 306.19 315.98 318.49 319.46 321.89

DA 66,028.5 306.36 316.33 319.02 320.11 322.61

DB 66,139.5 304.28 316.60 319.37 320.46 322.83

DC 66,344.5 306.08 316.94 319.60 320.66 322.95

DD 66,626.5 311.42 317.92 320.48 321.54 323.96

DE 66,928.5 316.36 323.30 325.06 325.97 328.28

DF 67,650.5 313.22 326.66 329.46 330.75 333.97

DG 68,732.5 302.68 327.83 331.22 332.75 336.35

DH 69,551.5 320.98 328.13 332.17 333.83 337.23

DI 69,782.5 321.52 332.55 334.91 335.88 341.14

DJ 70,007.5 323.77 334.03 336.77 337.77 342.35

DK 70,500.5 321.28 334.62 337.24 338.10 342.42

DL 70,812.5 321.91 334.65 337.34 338.21 342.46

DM 71,388.5 320.78 335.07 337.51 338.36 342.53

DN 71,616.5 319.60 335.11 337.52 338.38 342.54

DO 72,165.5 322.38 335.23 337.62 338.47 342.57

DP 74,512.5 320.25 336.44 338.67 339.51 342.99

DQ 74,550.5 320.25 336.59 338.89 339.76 343.15

DR 75,308.5 322.18 336.88 339.26 340.16 343.46

DS 76,282.5 322.43 337.30 339.79 340.77 344.09

DT 78,388.5 323.63 338.44 341.09 342.12 345.27

DU 79,630.5 321.38 339.12 341.62 342.62 345.64

DV 81,371.5 324.51 340.08 342.40 343.36 346.23

DW 83,918.5 326.19 340.88 343.19 344.17 346.97

1 Feet above confluence with Merrimack River.

2

 Backwater from Merrimack River.
Table 3 15

Table 3. Cross sections and the 2-, 5-, 25-year recurrence-interval and April 2007 flood data for Suncook River in Epsom, Pembroke, 

and Allenstown, NH.—Continued

[Location of cross sections A–DW are in appendix 4, where scale permits, and in appendix 1; ft, feet; NGVD 29, National Geodetic Vertical Datum of 1929]

Suncook River 

cross section

Cumulative 

distance upstream1

(ft)

Channel elevation 

(ft above NGVD 29)

Annual exceedance probability flood elevation

(ft above NGVD 29)

50 percent 

(2-year)

20 percent 

(5-year)

4 percent 

(25-year) April 2007

A 0 181.57 186.44 187.25 188.76 190.04

B 477 183.54 188.82 189.62 190.94 192.17

C 804 184.00 189.81 190.83 192.56 194.53

D 1,681 196.62 200.32 201.05 202.36 203.93

E 2,043 198.43 204.23 205.56 207.80 212.63

F 2,193 204.12 207.87 208.83 210.57 213.23

G 2,507 214.79 230.67 232.27 235.13 237.62

H 3,079 219.73 230.61 232.13 234.80 236.87

I 3,401 242.10 251.35 254.09 258.86 264.51

J 3,467 242.20 254.09 255.68 258.60 264.77

K 3,880 250.58 257.59 259.11 261.64 266.00

L 3,934 251.54 257.82 259.40 262.04 266.33

M 4,433 258.46 264.08 265.38 267.46 269.82

N 4,507 259.63 265.25 266.55 268.65 270.96

O 4,640 263.45 276.66 277.68 279.40 281.41

P 4,886 265.02 276.68 277.74 279.57 281.80

Q 5,126 265.07 276.81 277.95 279.94 282.30

R 5,283 263.79 276.83 277.99 280.02 282.42

S 5,575 269.45 276.85 278.02 280.07 282.53

T 5,860 268.32 276.75 277.82 279.67 281.86

U 6,045 270.65 276.77 277.78 279.49 281.44

V 6,172 268.84 277.54 278.93 281.41 286.91

W 6,935 274.18 279.14 280.08 281.82 287.19

X 6,945 274.18 279.88 280.97 282.87 287.31

Y 7,981 270.26 281.73 283.54 286.52 289.96

Z 9,331 267.96 282.14 284.17 287.58 291.34

AA 10,614 270.63 282.30 284.40 287.88 291.63

AB 11,542 270.97 282.46 284.67 288.31 291.99

AC 12,282 269.33 282.57 284.83 288.52 292.22

AD 15,482 266.90 282.96 285.36 289.20 292.98

AE 17,581 267.70 283.32 285.87 289.56 293.14

AF 19,123 268.58 283.52 286.17 289.95 293.56

AG 22,592 271.35 284.00 286.75 290.67 294.15

AH 23,529 270.36 284.37 286.96 290.75 294.18

AI 24,763 272.96 284.86 287.62 291.28 294.22

AJ 25,387 273.31 285.08 287.83 291.46 294.25

AK 26,696 273.13 285.51 288.23 291.79 294.16

AL 27,674 275.27 285.78 288.55 292.27 294.82

AM 28,253 277.21 286.00 288.78 292.53 295.24

AN 28,443 279.58 286.20 289.01 292.83 295.68

AO 28,766 281.21 287.39 289.71 293.30 296.18

AP 28,851 279.83 288.00 290.09 293.51 296.34

AQ 29,063 283.13 294.24 295.47 297.61 301.13

AR 29,088 283.13 294.24 295.47 297.62 301.15
16 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Table 3. Cross sections and the 2-, 5-, 25-year recurrence-interval and April 2007 flood data for Suncook River in Epsom, Pembroke, 

and Allenstown, NH.—Continued

[Location of cross sections A–DW are in appendix 4, where scale permits, and in appendix 1; ft, feet; NGVD 29, National Geodetic Vertical Datum of 1929]

Suncook River 

cross section

Cumulative 

distance upstream1

(ft)

Channel elevation 

(ft above NGVD 29)

Annual exceedance probability flood elevation

(ft above NGVD 29)

50 percent 

(2-year)

20 percent 

(5-year)

4 percent 

(25-year) April 2007

AS 29,216 280.40 294.25 295.48 297.59 301.04

AT 29,346.5 279.21 294.27 295.52 297.69 301.42

AU 29,599.5 279.10 294.27 295.51 297.64 301.22

AV 30,714.5 279.32 294.31 295.58 297.80 301.36

AW 31,746.5 276.97 294.39 295.73 298.09 301.88

AX 33,141.5 276.47 294.46 295.86 298.31 302.04

AY 34,226.5 279.64 294.54 296.02 298.63 302.44

AZ 34,926.5 277.09 294.58 296.06 298.61 302.36

BA 35,562.5 278.13 294.70 296.29 299.05 302.86

BB 36,077.5 278.99 294.75 296.37 299.17 303.00

BC 36,612.5 275.28 294.85 296.55 299.43 303.27

BD 37,932.5 280.62 295.03 296.84 299.89 303.84

BE 38,425.5 279.70 295.18 297.03 300.08 303.97

BF 39,120.5 277.56 295.34 297.27 300.41 304.33

BG 39,860.5 277.90 295.41 297.37 300.52 304.42

BH 40,505.5 284.98 295.53 297.54 300.80 304.76

BI 41,065.5 285.20 295.79 297.82 301.03 304.95

BJ 41,898.5 278.65 296.01 298.04 301.20 305.06

BK 43,018.5 284.04 296.28 298.36 301.54 305.36

BL 43,740.5 279.70 296.50 298.65 301.91 305.76

BM 44,820.5 285.93 296.71 298.90 302.22 306.07

BN 45,160.5 286.48 296.84 299.08 302.50 306.47

BO 45,723.5 278.21 297.00 299.33 302.85 306.75

BP 46,355.5 286.15 297.07 299.43 302.95 306.83

BQ 46,730.5 280.10 297.24 299.66 303.31 307.31

BR 47,374.5 286.45 297.37 299.83 303.57 307.61

BS 48,052.5 283.65 297.54 300.00 303.65 307.63

BT 48,852.5 282.55 297.77 300.28 303.98 307.97

BU 50,157.5 285.85 298.22 300.81 304.59 308.57

BV 50,623.5 286.80 298.37 300.99 304.86 308.94

BW 50,828.5 271.36 298.43 301.07 304.94 309.00

BX 51,080.5 287.32 298.40 301.03 304.87 308.90

BY 51,304.5 290.47 298.72 301.44 305.52 309.88

BZ 51,974.5 289.81 299.39 302.06 306.19 310.66

CA 52,462.5 289.73 299.64 302.34 306.47 310.94

CB 53,599.5 289.59 300.01 302.52 306.58 311.04

CC 54,414.5 293.58 300.59 302.80 306.67 311.09

CD 55,868.5 292.55 301.51 303.00 306.72 311.12

CE 56,344.5 294.28 301.59 303.04 306.73 311.13

CF 56,879.5 294.58 301.88 303.15 306.75 311.14

CG 57,635.5 295.63 301.99 303.19 306.75 311.14

CH 58,326.5 296.89 302.29 303.36 306.78 311.15

CI 58,366.5 296.89 302.33 303.40 306.80 311.16
Table 3 17

Table 3. Cross sections and the 2-, 5-, 25-year recurrence-interval and April 2007 flood data for Suncook River in Epsom, Pembroke, 

and Allenstown, NH.—Continued

[Location of cross sections A–DW are in appendix 4, where scale permits, and in appendix 1; ft, feet; NGVD 29, National Geodetic Vertical Datum of 1929]

Suncook River 

cross section

Cumulative 

distance upstream1

(ft)

Channel elevation 

(ft above NGVD 29)

Annual exceedance probability flood elevation

(ft above NGVD 29)

50 percent 

(2-year)

20 percent 

(5-year)

4 percent 

(25-year) April 2007

CJ 59,098.5 296.17 302.54 303.59 306.85 311.18

CK 59,428.5 296.20 302.75 303.73 306.86 311.17

CL 59,963.5 296.75 303.61 304.56 307.08 311.25

CM 60,519.5 299.31 304.51 305.34 307.25 311.28

CN 61,159.5 298.38 305.06 305.41 307.10 311.27

CO 61,867.5 299.16 307.55 309.17 311.07 312.23

CP 62,546.5 299.17 308.58 310.30 312.19 313.75

CQ 63,034.5 299.49 309.20 311.00 313.13 315.03

CR 63,474.5 300.55 309.85 311.80 314.21 316.31

CS 63,794.5 301.23 310.33 312.33 314.94 317.11

CT 64,131.5 302.17 310.49 312.42 314.93 316.82

CU 64,399.5 303.38 310.80 312.74 315.33 317.41

CV 64,631.5 303.48 311.17 313.14 315.92 318.39

CW 64,892.5 304.21 311.35 313.29 316.10 318.67

CX 65,149.5 303.58 311.79 313.63 316.34 318.92

CY 65,882.5 306.19 313.08 314.82 317.52 319.94

CZ 65,892.5 306.19 313.07 314.79 317.46 319.83

DA 66,028.5 306.36 313.36 315.11 317.90 320.53

DB 66,139.5 304.28 313.53 315.34 318.21 320.86

DC 66,344.5 306.08 314.01 315.75 318.49 321.05

DD 66,626.5 311.42 315.35 316.79 319.39 321.94

DE 66,928.5 316.36 321.76 322.68 324.24 326.33

DF 67,650.5 313.22 323.80 325.48 328.23 331.26

DG 68,732.5 302.68 324.42 326.43 329.73 333.34

DH 69,551.5 320.98 326.12 327.17 330.49 334.46

DI 69,782.5 321.52 329.82 331.38 333.99 336.26

DJ 70,007.5 323.77 330.89 332.79 335.54 338.20

DK 70,500.5 321.28 331.34 333.31 336.19 338.48

DL 70,812.5 321.91 331.33 333.26 336.28 338.58

DM 71,388.5 320.78 331.88 333.87 336.49 338.73

DN 71,616.5 319.60 331.98 333.94 336.52 338.75

DO 72,165.5 322.38 332.14 334.07 336.63 338.83

DP 74,512.5 320.25 333.08 335.23 337.74 339.86

DQ 74,550.5 320.25 333.16 335.35 337.94 340.12

DR 75,308.5 322.18 333.40 335.60 338.28 340.52

DS 76,282.5 322.43 333.78 336.00 338.75 341.16

DT 78,388.5 323.63 334.77 337.07 340.00 342.53

DU 79,630.5 321.38 335.51 337.81 340.61 343.01

DV 81,371.5 324.51 336.47 338.83 341.45 343.75

DW 83,918.5 326.19 337.39 339.67 342.22 344.56

1

 Feet above confluence with Merrimack River.
18 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Table 4. Surveyed and HEC-RAS-modeled April 2007 elevations of flood high-water marks for the Suncook River, NH.

[HWM, high-water mark; ft, feet; NGVD 29, National Geodetic Vertical Datum of 1929; US, upstream; DS, downstream]

HWM 

name

Location of HWM 

relative to lettered 

cross sections

Latitude 

(N, 

decimal 

degrees)

Longitude 

(W, 

decimal 

degrees)

Town

Suncook 

field-surveyed 

elevation at 

HWM 

(ft above 

NGVD 29)

Suncook 

HEC-RAS 

model elevation at HWM 

(ft above 

NGVD 29)

Difference 

between 

field and 

model 

elevation 

(ft)

HWM 

rating HWM type

sunhwm6 81 ft US of G 43.12990 71.45518 Allenstown 

(Suncook)

236.22 237.51 1.29 Fair Seed line

sunhwm7 37 ft US of O 43.12938 71.44982 Allenstown 

(Suncook)

280.78 281.47 0.69 Fair Wash line

sunhwm9 30 ft US of V 43.13060 71.44550 Allenstown 

(Suncook)

285.11 286.92 1.81 Poor Seed line

sunhwm12 294 ft US of W 43.13219 71.44283 Allenstown 288.93 287.97 -0.96 Fair Wash line

sunhwm13 At AC 43.13660 71.43085 Allenstown 291.69 292.22 0.53 Good Silt line

sunhwm18 755 ft US of AE 43.14331 71.42523 Allenstown 293.05 293.08 0.03 Good Seed line

sunhwm20 83 ft DS of AO 43.15884 71.40630 Allenstown 297.17 296.05 -1.12 Good Seed line

sunhwm22 At DS face of 

abandoned 

Buck St. bridge

43.15957 71.40665 Allenstown 298.02 298.82 0.8 Fair Debris line

sunhwm23 At AQ 43.15967 71.40658 Allenstown 299.17 301.13 1.96 Fair Debris line

sunhwm26 At AU 43.16063 71.40569 Allenstown 299.85 301.22 1.37 Good Silt line

sunhwm27 58 ft US of AW 43.16594 71.40455 Allenstown 301.45 301.89 0.44 Good Seed line

sunhwm28 98 ft DS of AZ 43.17179 71.39785 Pembroke 302.33 302.36 0.03 Good Seed line

sunhwm29 At BA 43.17284 71.39644 Pembroke 302.65 302.86 0.21 Good Seed line

sunhwm30 215 ft US of BC 43.17599 71.39454 Pembroke 303.43 303.36 -0.07 Good Seed line

sunhwm31 70 ft DS of BM 43.18979 71.38192 Epsom 306.13 306.05 -0.08 Good Seed line

sunhwm32 At BT 43.19566 71.38341 Epsom 308.1 307.97 -0.13 Good Seed line

sunhwm33 At BX 43.20207 71.38300 Epsom 308.65 308.9 0.25 Good Seed line

sunhwm34 51 ft DS of BY 43.20224 71.38233 Epsom 309.50 309.66 0.16 Fair Wash line

sunhwm35 At BY 43.20235 71.38183 Epsom 309.81 309.88 0.07 Good Seed line

sunhwm36 54 ft DS of CH 43.20375 71.37122 Epsom 311.10 311.15 0.05 Fair Silt line

sunhwm38 At DI 43.22705 71.35659 Epsom 337.65 337.97 0.32 Good Seed line

sunhwm40 At DH 43.22618 71.35427 Epsom 335.48 334.46 -1.02 Fair Seed line
Table 5 19

Table 5. 1978 FIS and 2009 flood study 100-year flood elevations for selected locations in the Suncook 

River in Epsom, Pembroke, and Allenstown, NH.

[FIS, Flood Insurance Study; BFE, base flood elevation (100-year recurrence-interval flood); ft, feet; NGVD 29, National 

Geodetic Vertical Datum of 1929; US, upstream; DS, downstream; XS, cross section]

Location on 

flood-study base map

1978 FIS BFE 

elevation 

(ft above NGVD 29)

Flood study BFE 

elevation 

(ft above NGVD 29)

Difference between 1978 FIS 

and flood-study elevation 

(ft above NGVD 29)1

XS E 206 211.8 5.8

BFE 215 214 215 1

XS G 238 237.3 -0.7

US face of Main Street bridge 262 260 -2

BFE 270 269 270 1

55 ft US of XS O 284 281.2 -2.8

21 ft DS of BFE 285 284 285 1

XS V 286 286.4 0.4

320 ft US of BFE 290 289 290 1

320 ft US of XS AA 290 291.2 1.2

950 ft US of XS AD 291 292.4 1.4

XS AG 293 293.6 0.6

XS AK 294 293.6 -0.4

BFE 295 295 295 0

BFE 300 300 300 0

XS AT 305 301 -4

XS AX 306 301.6 -4.4

BFE 306 307 306 -1

BFE 307 307 307 0

XS BW 307 308.4 1.4

XS BX 308 308.3 0.3

US face of Short Falls Road bridge 309 308.5 -0.5

170 ft DS XS BZ 310 310 0

231 ft DS XS CM 311 310.5 -0.5

BFE 313 312 313 1

370 ft DS XS DH 339 334 -5

BFE 338 339 338 -1

BFE 339 340 339 -1

BFE 340 341 340 -1

600 ft US of XS DS 342 341.2 -0.8

XS DV 343 343.4 0.4

1

 A positive number means that the stream BFE is higher in this study than in the 1978 FIS, and a negative 

number means that the stream BFE is lower in this study than in the 1978 FIS.
20 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Table 6. Cross sections, 100-year recurrence-interval, and floodway data for Suncook River in Epsom, Pembroke, and 

Allenstown, NH.—Continued

[ft, feet; ft2, square feet; ft/s, feet per second; NGVD 29, National Geodetic Vertical Datum of 1929]

Flooding source Floodway Base flood water-surface elevation

Cross section

Cumulative 

distance 

upstream1

(ft)

Width

(ft)

Section area

(ft2)

Mean velocity

(ft/s)

Regulatory

(ft above 

NGVD 29)

Without 

floodway

(ft above 

NGVD 29)

With 

floodway

(ft above 

NGVD 29)

Increase

(ft)

A 0 387.5 1,707.0 9.2 2198.3 189.82 189.87 0.05

B 477 270.8 1,385.1 11.4 2198.3 191.97 191.97 0

C 804 322.9 2,507.8 6.3 2198.3 194.19 194.19 0

D 1,681 231.2 1,211.5 13.0 203.68 203.68 203.68 0

E 2,043 141.0 1,295.2 12.2 211.77 211.77 211.77 0

F 2,193 174.0 1,138.7 13.8 212.46 212.46 212.47 0.01

G 2,507 149.0 3,006.3 5.2 237.27 237.27 237.59 0.32

H 3,079 84.2 1,371.8 11.5 236.61 236.61 237.09 0.48

I 3,401 79.0 1,456.0 10.8 263.51 263.51 263.7 0.2

J 3,467 148.0 1,878.2 8.4 263.7 263.7 264.7 1.0

K 3,880 236.0 2,395.8 6.6 265.16 265.16 266.02 0.86

L 3,934 214.0 2,200.1 7.2 265.5 265.5 266.29 0.8

M 4,433 153.4 1,056.8 14.9 269.44 269.44 269.43 0

N 4,507 153.5 1,058.6 14.9 270.61 270.61 270.62 0

O 4,640 212.2 2,367.9 6.7 281.07 281.07 281.07 0

P 4,886 320.4 2,632.2 6.0 281.41 281.41 281.42 0.01

Q 5,126 380.8 4,350.7 3.6 281.9 281.9 281.9 0

R 5,283 451.0 5,645.4 2.8 282.01 282.01 282.01 0

S 5,575 920.0 7,809.2 2.0 282.1 282.1 282.1 0

T 5,860 241.0 1,930.3 8.2 281.49 281.49 281.49 0

U 6,045 187.5 1,307.8 12.0 281.11 281.11 281.12 0.01

V 6,172 149.0 2,321.1 6.4 286.44 286.44 286.49 0.05

W 6,935 149.0 1,492.9 10.0 286.71 286.71 286.80 0.09

X 6,945 149.0 1,505.7 9.9 286.8 286.8 286.89 0.09

Y 7,981 124.0 1,867.7 8.0 289.45 289.45 289.78 0.34

Z 9,331 181.4 2,798.5 5.3 290.76 290.76 291.15 0.39

AA 10,614 139.0 2,523.9 5.9 291.05 291.05 291.52 0.47

AB 11,542 186.0 3,107.5 4.8 291.42 291.42 292.12 0.70

AC 12,282 165.0 2,984.6 5.0 291.64 291.64 292.36 0.71

AD 15,482 933.0 9,180.9 1.6 292.39 292.39 293.18 0.79

AE 17,581 212.1 3,278.0 4.5 292.58 292.58 293.46 0.89

AF 19,123 155.0 3,042.2 4.9 292.99 292.99 293.93 0.94

AG 22,592 1,518.2 15,498.9 1.0 293.56 293.56 294.51 0.94

AH 23,529 1,536.6 13,447.5 1.1 293.59 293.59 294.53 0.94

AI 24,763 1,878.9 12,068.3 1.2 293.64 293.64 294.58 0.94

AJ 25,387 1,916.4 9,708.4 1.5 293.67 293.67 294.61 0.94

AK 26,696 144.7 2,403.1 6.2 293.63 293.63 294.57 0.94
Table 6 21

Table 6. Cross sections, 100-year recurrence-interval, and floodway data for Suncook River in Epsom, Pembroke, and 

Allenstown, NH.—Continued

[ft, feet; ft2, square feet; ft/s, feet per second; NGVD 29, National Geodetic Vertical Datum of 1929]

Flooding source Floodway Base flood water-surface elevation

Cross section

Cumulative 

distance 

upstream1

(ft)

Width

(ft)

Section area

(ft2)

Mean velocity

(ft/s)

Regulatory

(ft above 

NGVD 29)

Without 

floodway

(ft above 

NGVD 29)

With 

floodway

(ft above 

NGVD 29)

Increase

(ft)

AL 27,674 148.7 2,459.8 6.1 294.31 294.31 295.2 0.88

AM 28,253 188.0 2,683.1 5.5 294.71 294.71 295.64 0.93

AN 28,443 294.8 3,969.5 3.8 295.13 295.13 296.07 0.94

AO 28,766 354.6 3,307.1 4.5 295.65 295.65 296.56 0.91

AP 28,851 272.4 2,730.1 5.5 295.82 295.82 296.68 0.86

AQ 29,063 317.6 3,132.9 4.8 300.71 300.71 300.74 0.03

AR 29,088 333.6 3,325.5 4.5 300.73 300.73 300.75 0.02

AS 29,216 207.3 2,735.1 5.4 300.63 300.63 300.66 0.03

AT 29,346.5 208.0 2,730.1 5.4 300.95 300.95 300.96 0.01

AU 29,599.5 119.8 2,106.0 7.0 300.78 300.78 300.81 0.03

AV 30,714.5 103.1 1,857.6 7.9 300.92 300.92 300.97 0.05

AW 31,746.5 110.9 2,126.1 6.9 301.39 301.39 301.49 0.10

AX 33,141.5 137.2 2,468.1 5.9 301.54 301.54 301.96 0.42

AY 34,226.5 148.0 2,633.6 5.6 301.93 301.93 302.28 0.35

AZ 34,926.5 98.0 1,941.5 7.6 301.85 301.85 302.31 0.46

BA 35,562.5 147.6 2,753.6 5.0 302.33 302.33 303.09 0.76

BB 36,077.5 133.0 2,403.1 5.5 302.47 302.47 303.22 0.75

BC 36,612.5 161.0 2,985.4 4.4 302.73 302.73 303.62 0.89

BD 37,932.5 269.5 3,472.3 3.8 303.27 303.27 304.09 0.82

BE 38,425.5 264.8 3,516.5 3.7 303.41 303.41 304.31 0.90

BF 39,120.5 308.7 4,529.2 2.9 303.76 303.76 304.70 0.94

BG 39,860.5 516.5 5,213.7 2.5 303.84 303.84 304.84 1.00

BH 40,505.5 502.5 6,360.6 2.4 304.17 304.17 305.07 0.90

BI 41,065.5 543.9 6,693.7 2.1 304.36 304.36 305.28 0.92

BJ 41,898.5 413.1 5,144.0 2.6 304.48 304.48 305.47 0.99

BK 43,018.5 309.6 4,301.7 3.3 304.77 304.77 305.77 1.00

BL 43,740.5 260.5 4,250.1 3.1 305.16 305.16 306.14 0.98

BM 44,820.5 159.6 2,586.4 5.1 305.48 305.48 306.46 0.98

BN 45,160.5 228.7 3,015.4 4.3 305.87 305.87 306.81 0.94

BO 45,723.5 184.4 3,463.7 3.8 306.16 306.16 307.11 0.95

BP 46,355.5 140.1 2,426.9 5.4 306.23 306.23 307.19 0.96

BQ 46,730.5 212.2 3,490.0 3.8 306.70 306.70 307.61 0.91

BR 47,374.5 402.3 4,727.3 2.8 306.99 306.99 307.85 0.86

BS 48,052.5 189.6 3,168.9 4.1 307.02 307.02 308.00 0.98

BT 48,852.5 239.1 3,468.5 3.8 307.36 307.36 308.34 0.98

BU 50,157.5 240.8 3,255.5 4.0 307.95 307.95 308.86 0.91

BV 50,623.5 297.7 4,622.5 2.8 308.30 308.30 309.11 0.81
22 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Table 6. Cross sections, 100-year recurrence-interval, and floodway data for Suncook River in Epsom, Pembroke, and 

Allenstown, NH.—Continued

[ft, feet; ft2, square feet; ft/s, feet per second; NGVD 29, National Geodetic Vertical Datum of 1929]

Flooding source Floodway Base flood water-surface elevation

Cross section

Cumulative 

distance 

upstream1

(ft)

Width

(ft)

Section area

(ft2)

Mean velocity

(ft/s)

Regulatory

(ft above 

NGVD 29)

Without 

floodway

(ft above 

NGVD 29)

With 

floodway

(ft above 

NGVD 29)

Increase

(ft)

BW 50,828.5 302.7 8,158.7 1.6 308.37 308.37 309.26 0.89

BX 51,080.5 194.9 3,660.3 3.6 308.27 308.27 309.15 0.88

BY 51,304.5 156.6 2,020.0 6.2 309.19 309.19 309.95 0.76

BZ 51,974.5 243.3 3,484.7 3.6 309.94 309.94 310.52 0.58

CA 52,462.5 456.7 6,733.7 1.9 310.22 310.22 310.88 0.66

CB 53,599.5 509.7 6,345.9 2.0 310.32 310.32 311.05 0.73

CC 54,414.5 564.0 6,552.2 1.9 310.38 310.38 311.28 0.89

CD 55,868.5 2,183.0 24,461.2 0.5 310.40 310.40 311.40 1.00

CE 56,344.5 2,197.9 23,020.7 0.6 310.41 310.41 311.41 1.00

CF 56,879.5 2,699.9 28,973.8 0.4 310.42 310.42 311.42 1.00

CG 57,635.5 1,589.8 16,522.3 0.8 310.42 310.42 311.42 1.00

CH 58,326.5 1,291.8 14,011.0 0.9 310.44 310.44 311.44 1.00

CI 58,366.5 1,303.9 13,805.1 0.9 310.44 310.44 311.44 1.00

CJ 59,098.5 1,466.9 14,105.0 0.9 310.47 310.47 311.47 1.00

CK 59,428.5 818.6 7,457.9 1.7 310.46 310.46 311.46 1.00

CL 59,963.5 1,005.8 7,939.2 1.6 310.55 310.55 311.55 1.00

CM 60,519.5 914.4 6,572.4 1.9 310.58 310.58 311.58 1.00

CN 61,159.5 381.0 2,500.6 5.0 310.57 310.57 311.48 0.91

CO 61,867.5 345.5 2,096.5 6.0 311.82 311.82 312.82 1.00

CP 62,546.5 240.0 1,786.0 7.0 313.45 313.45 314.25 0.80

CQ 63,034.5 240.5 1,888.5 6.7 314.7 314.7 315.48 0.78

CR 63,474.5 251.1 2,187.9 5.8 315.98 315.98 316.82 0.83

CS 63,794.5 463.0 3,941.7 3.2 316.81 316.81 317.79 0.98

CT 64,131.5 154.0 1,726.3 7.3 316.58 316.58 317.58 1.00

CU 64,399.5 144.4 1,671.7 7.5 317.13 317.13 317.99 0.86

CV 64,631.5 123.0 1,499.2 8.4 318.02 318.02 318.24 0.22

CW 64,892.5 176.3 2,036.7 6.2 318.27 318.27 319.21 0.94

CX 65,149.5 183.6 1,963.3 6.4 318.52 318.52 319.51 0.99

CY 65,882.5 162.8 1,826.6 6.9 319.56 319.56 320.48 0.92

CZ 65,892.5 143.4 1,656.2 7.6 319.46 319.46 320.38 0.92

DA 66,028.5 149.0 1,883.5 6.7 320.11 320.11 320.81 0.70

DB 66,139.5 230.8 2,528.4 5.0 320.46 320.46 321.29 0.83

DC 66,344.5 156.2 1,856.2 6.8 320.66 320.66 321.44 0.78

DD 66,626.5 138.6 1,181.1 10.7 321.54 321.54 322.21 0.67

DE 66,928.5 133.0 1,034.3 12.2 325.97 325.97 325.97 0

DF 67,650.5 141.6 1,990.9 6.3 330.75 330.75 331.16 0.41

DG 68,732.5 119.7 1,972.9 5.2 332.75 332.75 333.02 0.27
Table 6 23

Table 6. Cross sections, 100-year recurrence-interval, and floodway data for Suncook River in Epsom, Pembroke, and 

Allenstown, NH.—Continued

[ft, feet; ft2, square feet; ft/s, feet per second; NGVD 29, National Geodetic Vertical Datum of 1929]

Flooding source Floodway Base flood water-surface elevation

Cross section

Cumulative 

distance 

upstream1

(ft)

Width

(ft)

Section area

(ft2)

Mean velocity

(ft/s)

Regulatory

(ft above 

NGVD 29)

Without 

floodway

(ft above 

NGVD 29)

With 

floodway

(ft above 

NGVD 29)

Increase

(ft)

DH 69,551.5 557.2 1,590.1 6.4 333.83 333.83 334.11 0.28

DI 69,782.5 921.3 3,512.6 2.8 337.5 337.5 337.5 0

DJ 70,007.5 700.5 2,653.5 3.7 337.77 337.77 337.77 0

DK 70,500.5 886.0 5,619.4 1.8 338.1 338.1 338.1 0

DL 70,812.5 843.0 4,793.1 2.1 338.21 338.21 338.21 0

DM 71,388.5 677.8 4,220.9 2.3 338.36 338.36 338.36 0

DN 71,616.5 670.0 4,870.1 2.0 338.38 338.38 338.49 0.11

DO 72,165.5 474.0 2,905.7 3.4 338.47 338.47 338.56 0.09

DP 74,512.5 109.0 1,466.0 6.7 339.51 339.51 339.98 0.47

DQ 74,550.5 170.0 2,019.8 4.9 339.76 339.76 340.36 0.60

DR 75,308.5 216.6 2,479.9 4.0 340.16 340.16 340.86 0.70

DS 76,282.5 223.6 2,645.5 3.7 340.77 340.77 341.57 0.80

DT 78,388.5 563.2 5,146.8 1.9 342.12 342.12 343.07 0.94

DU 79,630.5 604.8 4,846.4 2.0 342.62 342.62 343.61 0.99

DV 81,371.5 733.8 6,301.8 1.6 343.36 343.36 344.35 0.99

DW 83,918.5 307.4 3,246.9 3.0 344.17 344.17 345.16 0.99

1

 Feet upstream from confluence with Merrimack River (1 mile = 5,280 ft).

2

 Backwater from Merrimack River.
THIS PAGE INTENTIONALLY LEFT BLANK
Appendix 1 25

Appendix 1. Suncook River 10-, 2-, 1- and 

0.2-Percent Annual Exceedance Probability 

Flood Profiles (10-, 50-, 100-, and 500-Year 

Recurrence-Interval Floods, respectively)
26 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 1–1. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–1. Detailed cross-section information shown in table 2.

A B C D E

F

G H I

J

K

L

M

N

O

P Q

R

S T U

V

3303102902702502302101901701500.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

01

MERRIMACK RIVER BACKWATER

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

SEWER TRESTLE

CHINA MILL DAM

PEMBROKE DAM

SOUTH MAIN STREET

WEBSTER DAM

CANAL OUTLET

WEBSTER DAM

U.S. ROUTE 3 0.2

Annual exceedance probability flood,in percent

EXPLANATION

1210Streambed

Cross-section locationand identifier

Bridge deck

—

 high and low chord

C
Appendix 1 27 Figure 1–2. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–1. Detailed cross-section information shown in table 2.

330320310300290280270260250240

1.30 1.40 1.50 1.60 1.70 1.80 1.90 2.00 2.10 2.20 2.30 2.40 2.50 2.60

X

W Y Z AA AB

AC

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

ELEVATION, IN FEET (NGVD 29)

CONFLUENCE OF

HARTFORD BROOK

0.2

Annual exceedance probability flood,in percent

EXPLANATION

1210Streambed

Cross-section locationand identifier Z 02

CONFLUENCE OF

BAILEY’S BROOK

CONFLUENCE OF

BOAT MEADOW BROOK

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY
28 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 1–3. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–2. Detailed cross-section information shown in table 2.

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

03

 (NGVD 29) ELEVATION, IN FEET

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

0.2

Annual exceedance probability flood,in percent

EXPLANATION

110Streambed

Cross-section locationand identifier AD

2

3303203103002902802702602502402.60 2.70 2.80 2.90 3.00 3.10 3.20 3.30 3.40 3.50 3.60 3.70 3.80 3.90

AD AE

AF
Appendix 1 29 Figure 1–4. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–2. Detailed cross-section information shown in table 2.

330320310300290280270260250240

3.90 4.00 4.10 4.20 4.30 4.40 4.50 4.60 4.70 4.80 4.90 5.00 5.10 5.20

AG AH AI AJ

AK

CONFLUENCE OF

PETTINGILL BROOK

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

04

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

FEDERAL EMERGENCY MANAGEMENT AGENCY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

0.2

Annual exceedance probability flood,in percent

EXPLANATION

110Streambed

Cross-section locationand identifier AG

2
30 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 1–5. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–3. Detailed cross-section information shown in table 2.

3403303203103002902802702602505.20 5.30 5.40 5.50 5.60 5.70 5.80 5.90 6.00 6.10 6.20 6.30 6.40 6.50

AL AM AN AO

AP

AQ

AR

AS

AT

AU AV AW

AX AY

BUCK STREET DAM

ABANDONED BUCK

STREET BRIDGE

STATE ROUTE 28

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

05

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

0.2

Annual exceedance probability flood,in percent

EXPLANATION

110Streambed

Cross-section locationand identifier

Bridge deck

—

 high and low chord

AW

2
Appendix 1 31 Figure 1–6. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–4. Detailed cross-section information shown in table 2.

3403303203103002902802702602506.50 6.60 6.70 6.80 6.90 7.00 7.10 7.20 7.30 7.40 7.50 7.60 7.70 7.80

AZ BA BB BC BD BE BF

BG BH BI

DOWNSTREAM CORPORATE LIMIT -

EPSOM, NH

2

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

06

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

0.2

Annual exceedance probability flood,in percent

EXPLANATION

110Streambed

Cross-section locationand identifier BA

CONFLUENCE OF AMES BROOK

CONFLUENCE OF BEAR BROOK

CONFLUENCE OF FOWLER BROOK
32 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 1–7. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figures 4–4 and 4–5. Detailed cross-section information shown in table 2.

3403303203103002902802702602507.80 7.90 8.00 8.10 8.20 8.30 8.40 8.50 8.60 8.70 8.80 8.90 9.00 9.10

BJ BK BL BM BN BO BP

BQ BR

CONFLUENCE OF DEER BROOK

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

07

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

0.2

Annual exceedance probability flood,in percent

EXPLANATION

110Streambed

Cross-section locationand identifier BK

2
Appendix 1 33 Figure 1–8. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–5. Detailed cross-section information shown in table 2.

3503403303203103002902802702609.10 9.20 9.30 9.40 9.50 9.60 9.70 9.80 9.90 10.00 10.10 10.20 10.30

BS BT BU BV BW BX BY BZ CA

CB CC

10.40

SHORT FALLS ROAD

CONFLUENCE OF BURHAM BROOK

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

08

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

0.2

Annual exceedance probability flood,in percent

EXPLANATION

110Streambed

Cross-section locationand identifier

Bridge deck

—

 high and low chord

BT

2
34 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 1–9. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–5. Detailed cross-section information shown in table 2.

360350340330320310300290280270

10.40 10.50 10.60 10.70 10.80 10.90 11.00 11.10 11.20 11.30 11.40 11.50 11.60 11.70

CD CE CF CG C

C

H

I

CJ CK CL

CM CN

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

09

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

0.2

Annual exceedance probability flood,in percent

EXPLANATION

110Streambed

Cross-section locationand identifier CF

2

CONFLUENCE OF SUNCOOK WEST CHANNEL
Appendix 1 35 Figure 1–10. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–6. Detailed cross-section information shown in table 2.

360350340330320310300290280270

11.70 11.80 11.90 12.00 12.10 12.20 12.30 12.40 12.50 12.60 12.70 12.80 12.90 13.00

CO CP CQ CR CS CT CU CV CW CX CY

CZ

DA

DB

DC DD DE

DF

CONFLUENCE OF

LEIGHTON BROOK

CONFLUENCE OF

LITTLE SUNCOOK RIVER

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

10

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

0.2

Annual exceedance probability flood,in percent

EXPLANATION

110Streambed

Cross-section locationand identifier CQ

2
36 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 1–11. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figures 4–6 and 4–7. Detailed cross-section information shown in table 2.

370360350340330320310300290280

13.00 13.10 13.20 13.30 13.40 13.50 13.60 13.70 13.80 13.90 14.00 14.10 14.20 14.30

DG DH DI DJ DK DL DM DN DO

D

D

P

Q

DR

U.S. ROUTE 4

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

11

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

0.2

Annual exceedance probability flood,in percent

EXPLANATION

110Streambed

Cross-section locationand identifier

Bridge deck

—

 high and low chord

DH

2
Appendix 1 37 Figure 1–12. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figures 4–7 and 4–8. Detailed cross-section information shown in table 2.

390380370360350340330320310300

14.30 14.40 14.50 14.60 14.70 14.80 14.90 15.00 15.10 15.20 15.30 15.40 15.50 15.60

DS DT DU

DV

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

12

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

0.2

Annual exceedance probability flood,in percent

EXPLANATION

110Streambed

Cross-section locationand identifier DT

2
38 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 1–13. Suncook River 10-, 2-, 1-, and 0.2-percent annual chance flood profiles (10-, 50-, 100-, and 500-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–8. Detailed cross-section information shown in table 2.

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

13

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

0.2

Annual exceedance probability flood,in percent

EXPLANATION

110Streambed

Cross-section locationand identifier DW

2

390380370360350340330320310300

15.60 15.70 15.80 15.90 16.00 16.10 16.20 16.30 16.40 16.50 16.60 16.70 16.80 16.90

DW

UPSTREAM CORPORATE LIMIT - EPSOM, NH
Appendix 2 39

Appendix 2. Suncook River April 2007 Flood 

and 50-, 20-, and 4-Percent Annual Exceedance 

Probability Flood Profiles (2-, 5-, and 25-Year 

Recurrence-Interval Floods, respectively)
40 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 2–1. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–1. Detailed cross-section information shown in table 3.

3303102902702502302101901701500.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30

A B C D E

F

G H I

J

K

L

M

N

O

P Q

R

S T U

V

SEWER TRESTLE

CHINA MILL DAM

PEMBROKE DAM

SOUTH MAIN STREET

WEBSTER DAM

CANAL OUTLET

WEBSTER DAM

U.S. ROUTE 3

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

01

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY April 2007 flood EXPLANATION

450Streambed

Cross-section locationand identifier

Bridge deck

—

 high and low chord

C

20

Annual exceedance probability flood,in percent
Appendix 2 41 Figure 2–2. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–1. Detailed cross-section information shown in table 3.

330320310300290280270260250240

1.30 1.40 1.50 1.60 1.70 1.80 1.90 2.00 2.10 2.20 2.30 2.40 2.50 2.60

X

W Y Z AA AB

AC

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

02

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier Z

20

CONFLUENCE OF

BAILEY’S BROOK

CONFLUENCE OF

BOAT MEADOW BROOK

CONFLUENCE OF

HARTFORD BROOK

Annual exceedance probability flood,in percent
42 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 2–3. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–2. Detailed cross-section information shown in table 3.

330320310300290280270260250240

2.60 2.70 2.80 2.90 3.00 3.10 3.20 3.30 3.40 3.50 3.60 3.70 3.80 3.90

AD AE

AF

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

03

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier C

20

Annual exceedance probability flood,in percent
Appendix 2 43 Figure 2–4. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–2. Detailed cross-section information shown in table 3.

3303203103002902802702602502403.90 4.00 4.10 4.20 4.30 4.40 4.50 4.60 4.70 4.80 4.90 5.00 5.10 5.20

AG AH AI AJ

AK

CONFLUENCE OF PETTINGILL BROOK

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

04

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier AG

20

Annual exceedance probability flood,in percent
44 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 2–5. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–3. Detailed cross-section information shown in table 3.

340330320310300290280270260250

5.20 5.30 5.40 5.50 5.60 5.70 5.80 5.90 6.00 6.10 6.20 6.30 6.40 6.50

AL AM AN AO

AP

AQ

AR

AS

AT

AU AV AW

AX AY

BUCK STREET DAM

ABANDONED BUCK

STREET BRIDGE

STATE ROUTE 28

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

05

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier

Bridge deck

—

 high and low chord

AM

20

Annual exceedance probability flood,in percent
Appendix 2 45 Figure 2–6. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–4. Detailed cross-section information shown in table 3.

340330320310300290280270260250

6.50 6.60 6.70 6.80 6.90 7.00 7.10 7.20 7.30 7.40 7.50 7.60 7.70 7.80

AZ BA BB BC BD BE BF

BG BH BI

CONFLUENCE OF AMES BROOK

CONFLUENCE OF BEAR BROOK

CONFLUENCE OF FOWLER BROOK

DOWNSTREAM CORPORATE LIMIT -

EPSOM, NH

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

06

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier BA

20

Annual exceedance probability flood,in percent
46 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 2–7. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figures 4–4 and 4–5. Detailed cross-section information shown in table 3.

3403303203103002902802702602507.80 7.90 8.00 8.10 8.20 8.30 8.40 8.50 8.60 8.70 8.80 8.90 9.00 9.10

BJ BK BL BM BN BO BP

BQ BR

CONFLUENCE OF DEER BROOK

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

07

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier BK

20

Annual exceedance probability flood,in percent
Appendix 2 47 Figure 2–8. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–5. Detailed cross-section information shown in table 3.

3503403303203103002902802702609.10 9.20 9.30 9.40 9.50 9.60 9.70 9.80 9.90 10.00 10.10 10.20 10.30 10.40

BS BT BU BV BW BX BY BZ CA

CB CC

SHORT FALLS ROAD

CONFLUENCE OF BURHAM BROOK

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

08

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier

Bridge deck

—

 high and low chord

C

20

Annual exceedance probability flood,in percent
48 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 2–9. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–5. Detailed cross-section information shown in table 3.

360350340330320310300290280270

10.40 10.50 10.60 10.70 10.80 10.90 11.00 11.10 11.20 11.30 11.40 11.50 11.60 11.70

CD CE CF CG C

C

H

I

CJ CK CL

CM CN

CONFLUENCE OF SUNCOOK WEST CHANNEL

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

09

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier C

20

Annual exceedance probability flood,in percent
Appendix 2 49 Figure 2–10. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–6. Detailed cross-section information shown in table 3.

360350340330320310300290280270

11.70 11.80 11.90 12.00 12.10 12.20 12.30 12.40 12.50 12.60 12.70 12.80 12.90 13.00

CO CP CQ CR CS CT CU CV CW CX

CZ

CY

DA

DB

DC DD DE

DF

CONFLUENCE OF

LEIGHTON BROOK

CONFLUENCE OF

LITTLE SUNCOOK RIVER

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

10

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier CP

20

Annual exceedance probability flood,in percent
50 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 2–11. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figures 4–6 and 4–7. Detailed cross-section information shown in table 3.

370360350340330320310300290280

13.00 13.10 13.20 13.30 13.40 13.50 13.60 13.70 13.80 13.90 14.00 14.10 14.20 14.30

DG DH DI DJ DK DL DM DN DO

DP

DQ

DR

U.S. ROUTE 4

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

11

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier

Bridge deck

—

 high and low chord

DH

20

Annual exceedance probability flood,in percent
Appendix 2 51 Figure 2–12. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figures 4–7 and 4–8. Detailed cross-section information shown in table 3.

 390 380 370 360 350 340 330 320 310 300

14.30 14.40 14.50 14.60 14.70 14.80 14.90 15.00 15.10 15.20 15.30 15.40 15.50 15.60

DS DT DU

DV

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

12

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier DH

20

Annual exceedance probability flood,in percent
52 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 2–13. Suncook River April 2007 flood and 50-, 20-, and 4-percent chance flood profiles (2-, 5-, and 25-year recurrence-interval floods, respectively) in Epsom, Pembroke, and Allenstown, NH. Most, but not all, locations of cross sections are shown in appendix figure 4–8. Detailed cross-section information shown in table 3.

 390 380 370 360 350 340 330 320 310 300

15.60 15.70 15.80 15.90 16.00 16.10 16.20 16.30 16.40 16.50 16.60 16.70 16.80 16.90

DW

UPSTREAM CORPORATE LIMIT - EPSOM, NH

U.S. GEOLOGICAL SURVEY

EPSOM, PEMBROKE, AND ALLENSTOWN, NH

MERRIMACK COUNTY

STREAM DISTANCE, IN MILES ABOVE CONFLUENCE WITH MERRIMACK RIVER

13

ELEVATION, IN FEET (NGVD 29)

FLOOD PROFILES

SUNCOOK RIVER

April 2007 flood

EXPLANATION

450Streambed

Cross-section locationand identifier DH

20

Annual exceedance probability flood,in percent
Appendix 3 53

Appendix 3. Elevation Reference Marks
54 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Appendix 3. Elevation reference marks.

[ft, feet; NGVD 29, National Geodetic Vertical Datum of 1929; °, degrees; USCGS, U.S. Coast and Geodetic Survey; USGS, U.S. Geological Survey; 

NHDPWH, N.H. Department of Public Works and Highways]

Reference mark

Elevation 

(ft above 

NGVD 29)

Description of location

D 17 1934

(43° 07′ 48′′ N, 71° 26′ 48′′ W)

283.49 Standard USCGS disk stamped “D 17 1934” and set in the northeast corner 

of the U.S. Route 3 bridge, 19.0 ft northeast from the center of the road, 

4.0 ft northeast from the face of the bridge, and 1.5 ft southeast from the 

wingwall.

D 18 1934

(43° 08′ 30′′ N, 71° 25′ 05′′ W)

335.42 Standard USCGS disk stamped “D 18 1934” and set into a large outcrop, 

10.0 ft below the top of the outcrop and 4.0 ft above the ground, 15.0 ft 

above River Road. Located 84.0 ft south from a stone wall, 270 ft southeast from the center of River Road, 280 ft southeast from power pole 11⁄2 

NYNEX/PSNH/66/96.

D 19 1934

(44° 11′ 49′′ N, 71° 22′ 17′′ W)

354.28 Standard USGS disk stamped “D 19 1934” and set in a 25 × 25 ft partially 

exposed ledge, 10.0 ft above the road. Located 84.0 ft east-southeast from 

the center of River Road, 45.0 ft south-southwest from a stonewall, and 

103 ft east-northeast from the northeast corner of a white house. 

1149-0160

(43° 13′ 31.77′′ N, 71° 21′ 44.97′′ W)

368.09 Standard NHDPWH disk stamped “1149-0160” and set into the top of a 5-ft 

rock ledge 1.5 ft above the ground, 32.0 ft east from the center of the 

Rte. 28, 121 ft southwest from power pole CE/3A/CO, 112 ft south-southwest from the center of an iron-covered sewer manhole, 23.0 ft west from 

the woods line.

TBM 364 364.567

(43° 14′ 25′′ N, 71° 21′ 40′′ W)

364.57 Standard rivet set in an embedded boulder 15.0 ft above an abandoned railroad 

bed and 3.0 ft above the ground. Located 36.0 ft east from the abandoned 

railroad bed, approximately 500 ft west of Goboro Road and much lower in 

a wooded area behind a small field.
Appendix 4 55

Appendix 4. Suncook River Flood Map of the 

Floodway, and the 100- and 500-Year Flood 

from the Confluence with the Merrimack River 

to the Upstream Corporate Limit of Epsom, 

New Hampshire
56 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Index map to figures in Appendix 4.

Appendix Figure 4−1

Pembroke

Allenstown

Chichester

Epsom

Appendix Figure 4−2

Appendix Figure 4−3

Appendix Figure 4−4

Appendix Figure 4−5

Appendix Figure 4−6

Appendix Figure 4−7

Appendix Figure 4−8

Merrimack River

Soucook River

cnuS oo k R vi

e r

Limit of Study

Limit of Study

Route 28

bridge

Short Falls

Road

Round Pond

Bear Island

4

28

3

4

28

3

93

93
Appendix 4 57 Figure 4–1. Location of base, 500- and 100-year flood elevations and cross sections along the Suncook River in Epsom, Pembroke, and Allenstown, NH. Detailed cross-section information shown in appendix 1 and in tables 2 and 5.

EXPLANATION

Base flood elevationExtent of 100-year floodExtent of 500-year floodExtent of floodwaySuncook River cross section

Limit of detailed study

Limit of detailed study

290 200

265

215

270

285 260

210

250

290 200

265

215

270

285 260

210

250

292

A

B

Z

S

I

C R

AA

J

AC

D

AB

L

Q

F

Y

T P

V

O G

X U

E

K

M

N

W

A

B

Z

S

I

C R

AA

J

AC

D

AB

L

Q

F

Y

T P

V

O G

X U

E

K

M

N

W

71°26' 71°26'30" 71°27' 71°27'30"

43°8'

43°7'30"

0 1,000 2,000 FEET

0 250 500 METERS

500

Base Map is from aerial photogrammetry collected by Eastern Topographic, in May 2007,and May 2005, New Hampshire Department of Transportation tif Imagery.

Allenstown

Pembroke
58 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 4–2. Location of base, 500- and 100-year flood elevations and cross sections along the Suncook River in Epsom, Pembroke, and Allenstown, NH. Detailed cross-section information shown in appendix 1 and in tables 2 and 5.

Limit of Detailed Study

Limit of Detailed Study

AI

AJ

AH

AD

AG

AC

AK

AB

AE

AF

71°24' 71°24'30" 71°25' 71°25'30" 71°26'

43°9'

43°8'30"

EXPLANATION

Base flood elevationExtent of 100-year floodExtent of 500-year floodExtent of floodwaySuncook River cross section

0 1,000 2,000 FEET

0 250 500 METERS

500

Base Map is from aerial photogrammetry collected by Eastern Topographic, in May 2007,and May 2005, New Hampshire Department of Transportation tif Imagery.

292

Allenstown

Pembroke

293
Appendix 4 59 Figure 4–3. Location of base, 500- and 100-year flood elevations and cross sections along the Suncook River in Epsom, Pembroke, and Allenstown, NH. Detailed cross-section information shown in appendix 1 and in tables 2 and 5.

Limit of Detailed Study

Limit of Detailed Study 295

300

294

AJ

AI

AY

AW

AL

BA

AX

AZ

AV

AN

AH

AM

AP

AO

AK

AG

AU

AS

AQ AR AT

71°23'30" 71°24' 71°24'30" 71°25'

43°10'

43°9'30"

EXPLANATION

Base flood elevationExtent of 100-year floodExtent of 500-year floodExtent of floodwaySuncook River cross section

0 1,000 2,000 FEET

0 250 500 METERS

500

Base Map is from aerial photogrammetry collected by Eastern Topographic, in May 2007,and May 2005, New Hampshire Department of Transportation tif Imagery.

302

Allenstown

Pembroke
60 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 4–4. Location of base, 500- and 100-year flood elevations and cross sections along the Suncook River in Epsom, Pembroke, and Allenstown, NH. Detailed cross-section information shown in appendix 1 and in tables 2 and 5.

302

303

304

Limit of Detailed Study

305

306

307

BI

BG

BQ

BC

BA

BF

BD

BH

BP

BB

BJ

BE

BL

BM

BK

BN

BO

AZ

71°22'30" 71°23' 71°23'30" 71°24' 71°24'30"

43°11'30"

43°11'

43°10'30"

Allenstown

Pembroke

Epsom

Epsom

Base Map is from aerial photogrammetry collected by Eastern Topographic, in May 2007,and May 2005, New Hampshire Department of Transportation tif Imagery.

EXPLANATION

Base flood elevationExtent of 100-year floodExtent of 500-year floodExtent of floodwaySuncook River cross section

0 1,000 2,000 FEET

0 250 500 METERS

500
Appendix 4 61 Figure 4–5. Location of base, 500- and 100-year flood elevations and cross sections along the Suncook River in Epsom, Pembroke, and Allenstown, NH. Detailed cross-section information shown in appendix 1 and in tables 2 and 5.

306

307

310 310

308

311

312

CF

CI

CE

CD CG

CH

CC CB

BR

CA

BV

BW

BQ

BZ

BP

BX

CK

BT

BU

BS

CL

BO

CJ

CN

BY

CM

CO CP

71°22' 71°22'30" 71°23' 71°23'30"

43°12'30"

43°12'

43°11'30"

Epsom

Epsom

Base Map is from aerial photogrammetry collected by Eastern Topographic, in May 2007,and May 2005, New Hampshire Department of Transportation tif Imagery.

EXPLANATION

Base flood elevationExtent of 100-year floodExtent of 500-year floodExtent of floodwaySuncook River cross section

0 1,000 2,000 FEET

0 250 500 METERS

500
62 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 4–6. Location of base, 500- and 100-year flood elevations and cross sections along the Suncook River in Epsom, Pembroke, and Allenstown, NH. Detailed cross-section information shown in appendix 1 and in tables 2 and 5.

CU

CT

71°21' 71°21'30" 71°22' 71°22'30" 71°23'

43°13'30"

43°13'

43°12'30"

DI

DJ

CZ

DK

DL

CL

DE

CY

CX

DC

CW

CS

CQ

DD

CR

DB

DH

DG

CV

CO

CM

CN

CP

Limit of Detailed Study

320

330

335

325

DA

DF

315

320

330

335

335

325

318

338

EXPLANATION

Base flood elevationExtent of 100-year floodExtent of 500-year floodExtent of floodwaySuncook River cross section

0 1,000 2,000 FEET

0 250 500 METERS

500

311

312

313

317

323

327

332

333

Base Map is from aerial photogrammetry collected by Eastern Topographic, in May 2007,and May 2005, New Hampshire Department of Transportation tif Imagery.

Epsom

Epsom
Appendix 4 63 Figure 4–7. Location of base, 500- and 100-year flood elevations and cross sections along the Suncook River in Epsom, Pembroke, and Allenstown, NH. Detailed cross-section information shown in appendix 1 and in tables 2 and 5.

Limit of Detailed Study

340

335

335

338

339

342

Base Map is from aerial photogrammetry collected by Eastern Topographic, in May 2007,and May 2005, New Hampshire Department of Transportation tif Imagery.

EXPLANATION

Base flood elevationExtent of 100-year floodExtent of 500-year floodExtent of floodwaySuncook River cross section

0 1,000 2,000 FEET

0 250 500 METERS

500

DI

DJ

DN

DL

DQ

DP

DO

DK

DT

DR

DS

DH

DU

DM

71°21' 71°21'30" 71°22' 71°22'30"

43°14'30"

43°14'

Epsom Epsom
64 Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009

Figure 4–8. Location of base, 500- and 100-year flood elevations and cross sections along the Suncook River in Epsom, Pembroke, and Allenstown, NH. Detailed cross-section information shown in appendix 1 and in tables 2 and 5.

Limit of Detailed Study

344 DW

DT

DV

DU

71°21' 71°21'30" 71°22' 71°22'30" 71°23'

43°15'30"

43°15'

342

EXPLANATION

Base flood elevationExtent of 100-year floodExtent of 500-year floodExtent of floodwaySuncook River cross section

0 1,000 2,000 FEET

0 250 500 METERS

500

Base Map is from aerial photogrammetry collected by Eastern Topographic, in May 2007,and May 2005, New Hampshire Department of Transportation tif Imagery.

Epsom

Epsom

Chichester
Prepared by the Pembroke Publishing Service Center.

For more information concerning this report, contact:

Director

U.S. Geological Survey

New Hampshire-Vermont Water Science Center

331 Commerce Way, Suite 2

Pembroke, NH 03275

dc_nh@usgs.gov

or visit our Web site at:

http://nh.water.usgs.gov
Flynn—Flood Study of the Suncook River in Epsom, Pembroke, and Allenstown, New Hampshire, 2009—Scientific Investigations Report 2010–5127